I also was curious about this so I did some research.
It looks like age 20 to 34 has the lowest mortality rate. Older or younger than that has higher mortality.
And since 14 to 18 as a cohort are all minors, it’s completely reasonable that parents and society in general discourages this activity.
Taking risks at 35 and 14 are treated differently.
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.
I picked 14 and 35 for good reason. Both have a higher chance of mortality in pregnancy as a cohort.
Also 14 is relevant for the child marriage article, which is the current context.
18 year olds are not relevant to child marriage.
They are included in the statistics for "high risk" adolescent pregnancy in gp's reference which I take as a condemnation of both adult and minor teenage pregnancies and pregnancies in general.