There are a few ways to tell Tony Stewart is now a 40-year-old.
For one, the veteran has a few streaks of gray hair. He's now a team owner instead of just somebody's driver. And there are so many candles on his birthday cake that when he blows them out, he lives up to his nickname: Smoke.
But as of now, none of the signs that Stewart is 40 have shown up on the racetrack. He's still Tony Friggin' Stewart, the man whose peers voted him NASCAR's most talented driver last year.
As he gets older, will his skills someday decline? Or will Stewart walk away from the sport someday when he's still on top of his game?
"You don't know until you get in that position," he said. "Your heart and your mind might be 100 percent into it, but something (might) happen where you can't see as good or you don't feel as good in the car. Or you might be perfectly healthy and on top of your game and your mind says, 'I'm just tired of doing it.' You don't know what the variables are going to be."
Stewart once said that good drivers never forget how to drive and argued that the only reason a driver's performance may drop off is due to the car or circumstances on the team.
But when he said that, Stewart mentioned that eventually age would become a factor. After all, no one can race forever.
So I asked Stewart on Friday: How does a driver know when his career is starting to wind down?
"I think you just know," he said. "... I think mentally you know, your mind tells you something. It's no different than having a family or anything else – you just kind of know when the time is right. It's when priorities in your life start changing, that's when you can sense that. It's desire, it's health – there are a lot of variables to decide when that day really is that you walk away from it."
Certainly, I'm not saying Stewart has lost a step or is even close. Along with Jimmie Johnson, Kyle Busch, Jeff Gordon and perhaps Carl Edwards, Stewart is still among the drivers I'd want in my car more than any other.
But I wondered if Stewart still feels as good as he did when he was 20.
"Mentally? Yeah," he said. "Physically? Not so much."
Why doesn't he feel the same physically?
"I probably could take better care of myself," he replied.
Wasn't he on a good diet plan now? I asked sincerely.
He offered no response but smiled, gave me a friendly punch in the gut and walked into his hauler.