You're right, UA's average fleet age is 12.4 years while AA's average fleet age is 15 years. Not to worry about that, however, as the 230 new narrowbody planes on the way by 2017 will lower AA's average fleet age to the youngest among UA, DL and US.
Your implication is that AA's slightly older fleet means it needs more mechanics, on average. And you'd be right if AA hadn't already ordered replacements for most of its older narrowbodies. Starting six months from now, when first of the 130 Airbus and 100 additional 737s begin showing up, MD-80s will be heading to the desert at a rapid pace, along with the 762s and some of the older 757s.
