I do it will not eat babies, toddlers, children or elders. It will also not eat Sims with the unlucky, lucky, daredevil or good traits. I got that information off The Sims Wiki.
I've had them rejected for no reason at all. Sometimes I'll get a message that for some reason the Sim was not eaten but sometimes no message. Not to say the Wiki is wrong, it might be my game, but if I've fed a Sim with those traits to the plant more than once they sometimes get eaten. I've had them rejected for needing a shower, having the "loves the outdoors" trait and quite a few other reasons.
I've resorted to fencing in my plant, putting a locked gate on it and using the MOO cheat to move maids and paparazzi inside the fence. That's really who I wanted the plant to eat when I bought it.