the oldest driver in Nascar right now is 44. When did guys start retiring young in Nascar?

Harry Gant had the most wins of his career at 51. Retired at 54
Mark Martin, David Pearson, and Richard Petty retired at 54
Rusty Wallace and Bobby Allison retired at 50
Bill Elliot retired at 56
Terry Labonte retired at 57
Darrell Waltrip retired at 58
Cale Yarborough retired at 49
Dale Earnhardt wasn't ready to retire when he died at 50

http://www.foxsports.com/nascar/gallery/ranking-the-20-greatest-nascar-drivers-of-all-time-022112
http://www.racing-reference.info/driver.htm
http://www.postregister.com/articles/sports-pro/2017/04/26/nascar-stars-are-no-longer-driving-their-50s#

So why do they retire so much younger now?

Money, commitments, etc. They make far more than the old timers ever did, they have about the same number of races, but twice as many appearances and event commitments to sponsors as they do races. So, 40 races (for nice round even numbers) 90 appearances, and now you have 1/3 of the year accounted for, without any practice, development, testing, or travel.

From the interview between the Post Register and Dale Jr:

Earnhardt touched on the demands of the NASCAR schedule before the season, when he noted that he was doing nearly 90 sponsor or team commitments in addition to 38 races.

“Back in the ’70s and ’80s, when guys were racing into their 50s, they were running 28 races and had a lot of time off,” Earnhardt told The Associated Press in February. “They didn’t have sponsor responsibilities. Ninety days of work off the track? What was Bobby Allison doing in ‘83 with Miller? Twelve days off the track, maybe? They had a lot of time to do what they wanted to do to unplug and keep their battery charged.”

When Earnhardt announced Tuesday that he was retiring at the end of the year, Texas Motor Speedway President Eddie Gossage knew the immediate effect it would have on his gate.

“Dang it! Dale Jr. put my kids through college and I was hoping he would stick around long enough to send my grandkids to college,” Gossage said.