When do succubi mature?
The following is a panel from chapter 2 of the manga. It states that Astarotte will become an adult in the next 3 years.



Given that she's 10 at the start of the story, it seems that she'll be matured to the level of needing a harem by the age of 13. That would correspond to roughly the middle of puberty in human girls, so it seems likely that succubi mature at roughly the same rate as humans, though perhaps slightly faster.