It's possible to complete all careers with Discover University (DU) and you don't need to turn aging off. Sims were limited to one branch of a career before DU but university degrees can unlock the other branch, making it possible to complete all careers. There are plenty of ways to avoid aging without turning it off completely, including reward potions, spellcaster potions, and vampire immortality.
I think the most I've done with one Sim is around 5 or 6 careers. I just don't stick with any one save long enough to get through all of them.