On second thought...
I am reinstating the rule where you need to off your Elder slave. I think this adds "challenge" to the challenge. I found out that my painter sims are getting consistent masterpieces already. Having them create throughout their Elder state will be milking it, thus making the challenge not challenging at all.
Also, it gives the player an activity for their vampire. Killing an elder sim is easy when your vampire has the Control Emotion and Deprive Needs powers. The easiest method is using Control Emotion (and is more humane). You can do "Burst of Playfulness" and then "Burst of Happiness" have them drink a Happiness Potion.
So, Elders needs to be disposed off. >:)