Understandably so.

But what about 18 and 33?

Beyond rare risk of death to the mother, I think the health of the child to be born and the potential for younger siblings is an important consideration since we are talking about reproduction.

In Europe, marriage and pregnancies below 18 were rare and people did use to average 21 before "female education" as well but other cultures differed and differ and I don't know to what extent it is appropriate to have "global" organizations mess with their reproductive lives from a Western perspective whether it has 1820s views or 2020s views.