I have heard variously of Ty Cobb and Al Kaline being the youngest player–maybe just the youngest in the AL–to win a batting title. Ty Cobb was born December 18, 1886, and he was the AL’s champion batter in 1907 with an average of .350 (Macmillan). The 1907 season ended about 2 1/2 months before Cobb’s 21st birthday.
Al Kaline was born December 19, 1934, and he led the league in 1955 with an average of .340. And, similarly with Cobb, Kaline turned 21, 2 1/2 months after the 1955 season ended. So which one is younger? Is the last day of the season the criterion for reckoning a league-leading player’s age?
Kaline was born one day later, which would make him younger at any equivalent point in a season. In other words, on August 1 the year after both batters turned 20, Kaline would be one day younger.
Also, in 1955, the regular season ended on September 25. In 1907, the regular season ended on October 6. That made Kaline even younger than Cobb when the season ended – the date when the batting champion is declared.
Oh, OK. I see what you mean. So the last day of the season IS the point for such reckoning. Thanx. 
Just to muck things up more. Since 1900 wasn’t a leap year, you can subtract a day from Cobb’s age. So they would be alive the same number of days at a particular date.
Pete
Get out! There was really someone named “Al Kaline?”
Was he ever arrested for battery?
Yes Eve, there was/is an Al Kaline.