You're right. There are undoubtedly many teenage mothers. But we need to look at medians:

The median age for women of first age of birth in the United States is 27.6 years. The average "childbearing years" duration is 5.3 years. If we assume an age of 10 for average increase in independence, we get 27.6 + (5.3/2) + 10 == 40.25 years.

I’d think that demographic overlaps significantly with families that have more than one child. Significant for this questions is, when do folks have their last kid?