Why are you more or less finished at 30 when you play AFL?

Compared to the fit and healthy 20 year old, your body has been worked harder and may not have as much left in the tank - so your often written off. The example that Ricky Ponting sets (making a double century and then 88 in both innings recently) at the age of around 37 shows age isnt everything, but in the world of Aussie Rules players are so pressured to retire or always play well - I guess since clubs are undertaking many youth policies (recruiting/blooding young guns) they arent as happy with the older players taking up space on lists when they could be building for a better future.