Currently still playing ts3 and i've found that it is the same with that game as it is with ts4. If I have a game that I put on long lifespan, then switch to a different save game that I want to have normal lifespan, I have to go in to settings and change it back between each save. As far as I know aging length has never been save file specific. Maybe ts2/ts1 was but I can't remember.
That being said it would be nice if it was. Would save me a headache or two upon realizing my sims aren't aging as fast or at all because I forgot to switch the aging settings between saves. Have done this many times.