Luke was born in 19 ABY - which means that he is currently roughly 63 Standard Years old (IU) as at the end of "Apocalypse" (and interestingly, Mark Hamill will be roughly 63 when Episode 7 releases in 2015!). Luke has shown, IMHO, that he considers it his duty to fight for Peace and Justice and to fight against the agents and influence of the Dark Side. Also IMHO, Luke's natural inclination is to be in the front line, rather than a "manager" or a "chessmaster" - a Rear-Echelon Jedi, he is not! So, leaving aside the (hypothetical) life extending factors of advanced Health Care and the anti-aging benefits of the Force; when do you think that he would be ready/willing to step down and "Pass the baton" to the next generation of Jedi? Personally, I think that he would be thinking of handing over the reins sometime in the next five to ten years.