I really like the idea, but I'm afraid that the BoVD beat you to the punch. Under the alternative curses described for Bestow Curse, one of them is to advance the victim an age category with regard to their physical condition (and they thus do not benefit from mental ability bonuses). Since it is a curse, I'd also disqualify this from affecting dragons beneficially.
Given the lack of versatility of this spell and the long casting time, it might be worthwhile allowing this as a Sor/Wiz 4 and advancing the victim two age categories, with Fort negates. If you don't feel that it's worth level 4, add the clause that if aged past Venerable, the victim must make a second Fortitude save or die: not overpowered given that Phantasmal Killer is a one-action two-saves-or-die without having to bother advancing the victim through the age categories; alternately, Fort half (for one age category) would also beef it up to level 4.