We're trying to create a realistic rock game but as for age, we want to know how long a year is. Because people know players usually don't play for more than 40 hours until they get bored unless it's a really good game. So how quickly should they age. Perhaps the aging slows down after a set age? Or maybe it stops. Any suggestions?
Depends on how much content you have in the game.

If for example, you expect people to play for "20 hours" (the fairly standard length of a game these days) then make 1 year every hour.

So if your character starts at 20, at the end of the game they will be ending their main "career" at 40.

After that is extra, don't forget that they have a tendency to come back after a decade or two when their money runs out :p

If you don't feel you have enough content to fill that much, then make it shorter.
In response to Acebloke
Heh. I had a project where one game minute was equal to six real world seconds, with game years and game days a bit shorter than real world days and years (20 hours in a day and 205 days a year). If a player tried to play through their character's 20 game-year career, from their 20th to their 40th birthdays, it would have taken them nearly two years of real-world playtime. 660 days. 0_0

[edit] And if you only played for an hour a day, it would take you LONGER to complete you character's career than it would take to complete your own: 43 years 4 months, one week and 6 days (give or take a day, depending on how leap years fall).