Multporn

So basically the succubus is decade's to near an century or older, but due to being an relative failure,

when it comes to gathering lust energy, or something akin to it, her physical body had remined stagnated for the entire time.

Thus when the male protagonist had given said lust energy to the succubus, especially willingly, her evolution from,

Mini-Succubus, to an full on, complete one, had allowed her body to finally reach it's maturity, after all those decade's or so.