a very common, and a very stupid argument.
"they only did that because they had to" seems fine, except that Jacqui hasn't begun to examine whether they wanted to or not.
Why the assumption that the practice was undesirable?
Discussion
The glaring hole in the argument is that people who say that ancient humans "lived" only 40 years are wrong and mix up several basic concepts.
The most important one is "lifespan (longevity) vs average life expectancy". Ancient humans lived more or less the same number of years we do now. But deaths at birth and infancy were a lot higher, which is brings the average expectancy down by so much compared to now.
So no, having children in their teens didn't have anything to do with "seeing them grow up because we'll be dead by age 40".