I ran a custom encounter where my party had to use their life force (age) to restore the life and age to two Royal NPCs (drained by an evil magical entity as it can do this to creatures of royal blood). To restore the life force back to the two NPCs I allowed the condition that if they willingly give their life force the magical entity can medium the transfer at a much higher cost (4-8 years of their life = 1 of the Royals). Luckily they had a gnome who is now 210 years older.
After this my party is going to try to find ways to restore themselves to their previous age and I am preventing them from using the same method used before (as the evil entity sees no benefit in restoring them). This now has become a side quest for the party. I have not been able to find any high level spells or other methods of restoring lost age in the handbooks, and I am looking some ideas on how I might later give them a chance to restore them. I can create another custom encounter around this, but I don't want to make this easy for them.