The baron's self defeating purpose (spoilers)
So if the baron was waiting so eagerly for his daughter to come of age so he can start having sex with her, why was he constantly feeding her the serum? He was denying his own objective.
Unless he was a pleasure delayer like Tom Cruise in Vanilla Sky, which would basically make it the most gratifying fetish of all time (he waited 200 years for his release).
And besides, it seems that he was way past breeding and into pleasure territory, so don't tell me he had some sort of weird set of morals where he's ok with having sex with his daughter but only when she's reached puberty. Any sooner than that, and it seems to be a nobleman's faux pas.