I'm not talking about child marriage. Traditionally women were considered to be of marrying age at around 15 or 16 (note that this doesn't mean they got married at exactly this age -- it means the lower bound). The blog you link seems to be intentionally or unintentionally conflating child marriage (prepubescent girls) with marrying between 15 and 20. And indeed the sources it cites seem to largely support an average marrying age of around 20 for women until the 17th century.

This is also of course only for the medieval period, which is relatively recent all things considered. That still leaves 8500 years unaccounted for until the advent of agriculture, and during those 8500 years it's far more likely for women to have given birth to their first child younger rather than older than in the medieval period.