Well, I haven't much experience in 2e, but in 1e (showing my age here) the potion of longevity (basically de-aged you 5 years) had the component requirements of A.) a pint of elf blood for the arcane version or B.) The hair of a saint for the devine version. In 1e, they were pretty expensive. If I were to make a 3e version, I would base it off of a fairly high spell, say limited wish. I am not aware of any spell that specifically removes age, but I think a limited wish would do the trick.
Greythax.