[Exxar]

If I properly understand that the sole point and effect of this is for the character not to age if he devours souls, then it's just a simple Unaging disadvantage with an Accessibility modifier whose value depends on how frequently he's in need of fresh souls. This would, however, result with the character resuming his natural aging during the period where he doesn't have access to souls, which isn't much of an inconvenience.

Perhaps a better treatment would be to purchase Unaging together with Dependency, and I'd say it would be fair for him to age a year instead of losing a point of HP, without any modifications to cost.