I've never done this, but I know that the cowplant has to be hungry, and have the cake out in order to eat anybody. When a Sim is eaten for the first time, whatever mood they are in at the time will be drained by the cowplant, and milking the cowplant will provide an essence giving that mood for 12 hours. In order to get an essence that reverses aging, the same Sim would need to be eaten a second time within 48 hours.
From what I hear, it's very hard to get the cowplant to eat NPCs.