To my understanding, if one mother birthed 3 children at ages 15, 20, and 25, then her age of conception would be 20. So it could be the case that women did become mothers in their teens if they also continued bearing children into their late 20s.
And if they got more successful at bringing those children to adulthood with each try (a pattern frequently seen in animals), the visible average resulting from that 15, 20, 25 example would even be higher than 20.
