^^^ i think the best way to measure age if you have a little extra time is to weight each player's age by the percentage of the team's total minutes he plays. still not perfect- doesn't take into account injuries, or perfectly reflect rookies who will be key players in a few season. but if you want to talk numbers not sure if you can do much better.
having a little extra time at work

, i ran the knicks' numbers and got a weighted average age of 25.7 years. not gonna do the whole league, but will do a few sample teams when i get a chance. interesting teams could be grizz celtics (true lottery/rebuilding teams) spurs heat (win now teams) mavs suns (top flite teams with youth). requests?
finally having some fun with Excel...