Sam says Cameron IS writing the novel (the Avatar bible)

[Human No More]

Yes - Jake's avatar is fully grown then they arrive and was clearly not when they left judging by conversation ("wow, they've got big" "yeah, they fully mature on the flight out") - so presumably, it is 6-7 years or so. I would guess that the lack of activity and injury combined with the nutrients in the tank would mean that the effective age counts for less against remaining lifespan than it would if it had actually been lived at normal speed.