Prince George Alexander Louis

Prince George, accompanied by Prince William, meets with head teacher Helen Haslem on his first day at Thomas's, a prep school in London, on Sept. 7, 2017.

Prince George, with Prince William and head teacher Helen Haslem on his first day at Thomas's, a prep school in London, on Sept. 7, 2017.

Prince William takes Prince George to his first day at Thomas's, a prep school in London, on Sept. 7, 2017.

Prince George arrives at Thomas's, a prep school in London, for his first day of school on Sept. 7, 2017.

Prince George arrives at Thomas's, a prep school in London, for his first day of school on Sept. 7, 2017.

Prince George of Cambridge views helicopter models before departing from Hamburg airport on the last day of an official visit to Poland and Germany on July 21, 2017.

Prince George holds the hand of his father, Prince William, upon their arrival at the airport in Berlin on July 19, 2017.

Prince George poses for a portrait to celebrate his fourth birthday at Kensington Palace in London at the end of June 2017.

Prince George waves as he leaves in a car after attending the wedding of his aunt Pippa Middleton to James Matthews at St. Mark's Church in Englefield, England, on May 20, 2017.

Prince George of Cambridge waves as he leaves from Victoria Harbour to board a sea-plane on the final day of their Royal Tour of Canada on October 1, 2016 in Victoria, Canada.

Prince George leaves Christmas Day service at St Mark's Church in Englefield, England, on Sunday, Dec. 25, 2016.

Prince George of Cambridge presses his face against the window as he leaves from Victoria Harbour aboard a sea-plane on the final day of their Royal Tour of Canada on October 1, 2016 in Victoria, Canada.

Catherine, Duchess of Cambridge, and Prince George attend a party at Government House in Victoria, British Columbia, Canada, on Sept. 29, 2016.

Prince George plays with bubbles at a children's party for Military families at Government House in Victoria, British Columbia, Canada, on Sept. 29, 2016.

Prince George attends a children's party for Military families at Government House in Victoria, British Columbia, Canada, on Sept. 29, 2016.

Prince George on his first day at the Westacre Montessori nursery school near Sandringham in Norfolk, eastern England, on Wednesday, Jan. 6, 2016.

Prince George on his first day at the Westacre Montessori nursery school near Sandringham in Norfolk, England on Jan. 6, 2016.

Britain's Prince George at Sandringham in Norfolk, England in a recent but undated photo issued by William and Kate, the Duke and Duchess of Cambridge, on Friday, July 22, 2016, Prince George's third birthday.

Britain's Prince George at Sandringham in Norfolk, England in a recent but undated photo issued by William and Kate, the Duke and Duchess of Cambridge, on Friday, July 22, 2016, Prince George's third birthday.

Recent but undated photo issued on Friday, July 22, 2016, by William and Kate, the Duke and Duchess of Cambridge, of Britain's Prince George at Sandringham in Norfolk, England.

Recent but undated photo issued on Friday July 22, 2016, by William and Kate, the Duke and Duchess of Cambridge, of Britain's Prince George with the family dog Lupo, at Sandringham in Norfolk, England.

Prince George visits the Royal International Air Tattoo at RAF Fairford with his mother Catherine, Duchess of Cambridge on July 8, 2016 in Fairford, England.

Catherine, Duchess of Cambridge, Prince William, Duke of Cambridge and Prince George during a visit to the Royal International Air Tattoo at RAF Fairford on July 8, 2016 in Fairford, England.

Prince William, Duke of Cambridge and Prince George during a visit to the Royal International Air Tattoo at RAF Fairford on July 8, 2016 in Fairford, England.

Prince George waves during a visit to the Royal International Air Tattoo at RAF Fairford on July 8, 2016 in Fairford, England.

Britain's Prince George meets with U.S. President Barack Obama, center, and first lady Michelle Obama, at Kensington Palace in London on Friday April 22, 2016.

Prince George, center, talks to his mum, Kate, the Duchess of Cambridge after meeting President Barack Obama and first lady Michelle Obama, at Kensington Palace in London on Friday, April 22, 2016.

This image made available by Kensington Palace Saturday, June 6, 2015, taken by Kate, Duchess of Cambridge, at Amner Hall, eastern England in mid-May 2015 shows Britain's Princess Charlotte, right, being held by her brother, 2-year-old, Prince George.

Britain's Prince George arrives with his father, Prince William, Duke of Cambridge, at the Lindo Wing to visit his mother, Kate Middleton, Duchess of Cambridge, and newborn sister, Princess Charlotte Elizabeth Diana, at St. Mary's Hospital in Paddington, west London, on Saturday, May 2, 2015.

Britain's Prince George arrives with his father, Prince William, Duke of Cambridge, at the Lindo Wing to visit his mother, Kate Middleton, Duchess of Cambridge, and newborn sister, Princess Charlotte Elizabeth Diana, at St. Mary's Hospital in Paddington, west London, on Saturday, May 2, 2015.

Britain's Prince George arrives with his father, Prince William, Duke of Cambridge, at the Lindo Wing to visit his mother, Kate Middleton, Duchess of Cambridge, and newborn sister, Princess Charlotte Elizabeth Diana, at St. Mary's Hospital in Paddington, west London, on Saturday, May 2, 2015.

Britain's Prince George arrives with his father, Prince William, Duke of Cambridge, at the Lindo Wing to visit his mother, Kate Middleton, Duchess of Cambridge, and newborn sister, Princess Charlotte Elizabeth Diana, at St. Mary's Hospital in Paddington, west London, on Saturday, May 2, 2015.

One of the three official Christmas images showing Prince George in a courtyard at Kensington Palace in central London.

One of the three official Christmas images showing Prince George in a courtyard at Kensington Palace in central London.

One of the three official Christmas images showing Prince George in a courtyard at Kensington Palace in central London.

Britain's Prince George walks during a visit to the Sensational Butterflies exhibition at the Natural History Museum in London on July 2, 2014.

Catherine, Duchess of Cambridge, and Prince George of Cambridge leave Fairbairn Airbase as they head back to the UK after finishing their Royal Visit to Australia on April 25, 2014, in Canberra, Australia.

Catherine, Duchess of Cambridge, Prince William, Duke of Cambridge, and Prince George of Cambridge leave Fairbairn Airbase as they head back to the UK after finishing their Royal Visit to Australia on April 25, 2014, in Canberra, Australia.

Australian Prime Minister Tony Abbott bidding farewell to Britain's Prince William, Duke of Cambridge, and Catherine, Duchess of Cambridge, as she holds baby Prince George before boarding a plane in Canberra, Australia, on April 25, 2014.

Britain's Catherine, Duchess of Cambridge, and Britain's Prince William, Duke of Cambridge, watch their son George look at an Australian animal called a bilby during a visit to the Taronga Zoo in Sydney, Australia, on April 20, 2014.

Catherine, Duchess of Cambridge, holds Prince George of Cambridge as they look at a Bilby called George at Taronga Zoo on April 20, 2014 in Sydney, Australia.

Catherine, Duchess of Cambridge, holds Prince George of Cambridge as they visit the bilby enclosure at Taronga Zoo on April 20, 2014, in Sydney, Australia.

Britain's Prince William, Duke of Cambridge, holding his son Prince George of Cambridge as they board a Royal Australian Airforce Jet heading to Sydney, Australia, at an airport in Wellington, New Zealand, on April 16, 2014.

Britain's Prince George playing with the toys during a Plunket nurse-and-parents group visit at Government House in Wellington, New Zealand, on April 9, 2014.

Catherine, the Duchess of Cambridge, holds Prince George as she and Prince William arrive at Wellington Airport in New Zealand on Monday, April 7, 2014.

Britain's Catherine, Duchess of Cambridge, holds her son, Prince George of Cambridge, as they arrive at Wellington Military Terminal in Wellington, New Zealand, on April 7, 2014.

Prince William and Kate talk to Queen Elizabeth II as they arrive, holding their son Prince George, at Chapel Royal in St. James's Palace in London on Oct. 23, 2013.

Prince William, Duke of Cambridge arrives, holding his son Prince George, at Chapel Royal in St. James's Palace, before the baby's christening by the Archbishop of Canterbury in London on Oct. 23, 2013.

Prince George of Cambridge is carried by Prince William, Duke of Cambridge, as they arrive at Chapel Royal in St James's Palace ahead of the christening by the Archbishop of Canterbury on Oct. 23, 2013, in London, England.

Catherine, Duchess of Cambridge, and Prince William, Duke of Cambridge, pose for a photograph with their son, Prince George Alexander Louis of Cambridge, surrounded by Lupo, the couple's cocker spaniel, and Tilly the retriever (a Middleton family pet) in the garden of the Middleton family home in August 2013 in Bucklebury, Berkshire.

Britain's Prince William, Duke of Cambridge, and his wife, Catherine, the Duchess of Cambridge, depart the Lindo Wing of St. Mary's hospital with their child, Prince George, in London, Britain, on July 23, 2013.

Catherine, Duchess of Cambridge, holds her newborn baby boy St. Mary's hospital in London, Britain, on July 23, 2013.