I think age, in and of itself, is irrelevant. If one really, truly desires to transition, it makes no difference...
Honestly, if a person comes up with reasons not to transition, then its probably an accurate reflection of their level of need/motivation. For example...I'll use myself. In my "ideal" world, I'd live full time. And yet I often consider my family, age and physical factors as justification for leaving things as they are. (I spend about 70% of my life en femme.) Still, when you get down to it, I just don't have a compelling need or desire to go all the way.