Giants head coach Tom Coughlin reviews a play on the...

Giants head coach Tom Coughlin reviews a play on the sidelines during a game against the Tennessee Titans at LP Field on Dec. 7, 2014 in Nashville. Credit: Getty Images / Frederick Breedon

Take it from a Hall of Fame coach who's old enough to be Tom Coughlin's father: If the Giants factor in Coughlin's age when deciding the coach's fate beyond this season, they're making a big mistake.

"Chronological age is only an approximation of your functional age," former Bills coach Marv Levy said from his home in Chicago on Thursday. "Tom is older than most of the guys coaching now, but he's still going great. Age, to me, is just a minor factor."

Levy, who turned 89 in August -- the same month Coughlin turned 68 -- speaks from experience. He didn't retire from the Bills until he was 72 and quickly came to regret that decision because he realized he still had an abiding love for the game. Levy sees no reason Coughlin can't continue coaching as long as he chooses -- or as long as the Giants let him.

It's uncertain whether the Giants (4-9) will retain Coughlin beyond this season, the fifth time in the last six years that they haven't reached the playoffs. But those two Super Bowl championship rings make the decision more complicated.

"My only advice for Tom is to do what your heart tells you," said Levy, who has known Coughlin for several years. "If he has the passion and love for the game, there's no reason to stop coaching."

Coughlin has refused to address his situation as far as next season is concerned; he wants the focus to remain on his players and how they prepare for their final three games. But he has shown zero -- and I mean zero -- signs of slowing down. He still works out early each morning, and often at the end of the day, too. He still shows the same enthusiasm in practice and on game day. And he still has a firm grasp of the concepts that make a team successful.

But he does occasionally offer some self-deprecating humor about his age. A few days ago, for instance, Giants public relations director Pat Hanlon made a remark near the end of a news conference about Coughlin's high energy level. Coughlin smiled, and as he left the news briefing, he quipped, "Every time I look in the mirror, I think, 'Who is that old [guy]?' ''

When Coughlin was reminded two weeks ago that he was about to coach his 300th game, he said, "For a young guy, it's hard to believe it's 300 games. I must have started when I was 15." (He actually was 49 when he coached his first game for the Jaguars in 1995.)

The fact that he is nearing the end of a second straight losing season has raised the possibility of his dismissal. With a roster in transition, Coughlin's age has been brought up as a potential factor in whether the Giants want to keep him.

"Tom is still in good shape, still loves coaching, and it looks like the passion for the game is still there," said Levy, who still runs for 30 minutes a day and works out with weights.

Levy, who led the Bills to four straight Super Bowls in the 1990-93 seasons, losing them all, regrets not coaching longer. He admitted the grind of the NFL got to him by 1997, when he stepped down after a 6-10 season.

"I'd been there a long time, and we weren't quite as good as we had been before," he said. "It looked like it would take a few years to get back to that, and I just felt I needed a break."

Within two years -- at age 74 -- he wanted to coach again.

"Two or three years after I did retire, there was an owner -- I won't tell you who he is -- who asked me to come in as a consultant," Levy said. "During the conversation, I said I'd be very interested in being the head coach. He said, 'Marv, I'm 15 years younger than you, and I couldn't even do that.' ''

Levy returned to Buffalo in 2006 to become the team's general manager -- at age 80. When coach Mike Mularkey resigned a week later, Levy called team owner Ralph Wilson. "I said to Mr. Wilson that I'd be very interested in coaching the team," Levy said. "He said, 'Marv, I hired you as the general manager.' And I left it alone after that."

As far as Levy is concerned, age really is just a number. The Giants ought to remember that when considering Coughlin's future. With the energy and enthusiasm of a man half his age, and with not a hint of slowing down, it shouldn't factor into the equation.

Coughlin plans to stick around a lot longer. We'll find out if it'll be with the Giants.