With our(northern Europe) crashing fertility rate there's now also discussions about adding on "when the woman is 25 this happens and you're this likely to get pregnant, at 30 it's like this...", just so that people can plan and try for the family they want. If one wants 3 kids and don't want IVF you should apparently start around when the woman is 25-28 or something like that?
But who's financially secure at 25?