I BE THE NECROMANCER! *raises arms*
Having re-read this thread after... a frightening amount of years, especially as the thread's subject is starting to affect me already...
, I must say I like the idea of having anti-aging spells be spread out in the gaps of existing spell lists. I am going to explore the lists for that a bit in the following, asking for opinions. I'll use RMFRP for this, as it currently is the rule system I use in my campaign.
Essence has its own lists in the Essence Companion, namely Brewing Lore (closed) and Sustain Body (open). It stands to reason that similar things could be achieved with Essence, but the general notion of Essence is that it doesn't do healing and manipulation of life very good. Other possible candidate lists for adding some sort of aging mitigation magic would be Altering Living Things (closed) or Physical Improvements (open) from Spell Law (working the with German translation here, so the actual names may be slightly different).
Channeling is depicted in the books as the one magic source most suited for all kinds of healing, so it stands to reason that most powerful anti-aging magic should be found in this realm. For placing such spells in them, I would propose Ways Of Healing (open), Law of Blood (closed), and possibly the other closed healing lists (Law of Bones, Law of Muscles, etc.) in Channeling whose anti-aging spells would focus on specific ailments from aging related to their subject organs. In the Channeling companion, some version of anti-aging might be doable with the Time Lore list, while Death Mastery and Necromancy could possibly allow some sort of reverse spell somewhere in there, depending on the god.
Mentalism would be more limited - only affecting the caster himself, I guess. Fitting lists would be Self-Healin (open), and some Lay Healer Base lists. Mentalism-Companion also has a list named Medical Law, which should work as a place for high-level, medium-power anti-aging spells.
Out of the dual-realm professions, I would assume the Healer's lists to containing similar anti-aging spells as the closed Channeling lists.
The Arcane list in this thread however, well, I just like it too much to not ALSO have it in a game world.