In the main-menu or ingame you have to open the options and go to 'Mod Settings' -> open 'Children, school and learning':

'Child age' -> how old they are born
'Base pregnancy chance' -> how likely a pregnancy is (lower value=less chance, 100=would be always[if they are able to have children])

'Learn min. age' -> when colonists can start to learn
'Learn max. age' -> when colonists cannot learn anymore

Other interesting ones:

'Teach min. age' -> how old someone has to be to teach

'Modifier: Pregnancy duration' -> Defines how long the pregnancy duration until birth is (100=default value = 45 days for a human --- 50=would be half the time etc.)

Thank you so much!! I've been weeks searching for this!! I was searching this in the Mods tab... That's why I couldn't find it

no problem ^^