Though you could argue that (if the Extremis really does fix damage on a cellular level, like they've said in canon in a couple of places) the Extremis has the ability to confer Logan-level insanely long lifespans. You could argue that age-related damege, since it occurs gradually and slowly, could be healed just as easily by a low-level healing factor like the Super Soldier Serum's or the Extremis as by a high-level one. I.E. that the gradual damage of aging can slowly heal as it occurs so that you're in a holding pattern, but that injuries, which occur all at once and suddenly, are harder to heal.

There's no evidence either way, since nobody in Marvel has gotten old yet who wasn't old to start with (like Aunt May or Professor X), so you could pretty much do whatever you wanted with it.