Statistically, most sex over the course of human history has been teenage sex. It is only in the past 1.5 centuries that the pressure to be over the age of 18 to 20 has been present. It used to be that if somebody waited that long, they were considered a spinster.
that's not correct at all. People sometimes became betrothed younger than now, but that was more a family-family situation, as you all lived crammed into a large multi-generational household. Also, historic age of puberty was 16-17 and only started declining after WWI. the idea of fertile teenagers only makes sense with 20th century+ nutrition.
There's some sample data here of 18th-19th century marriage ages, in France and Canada, they're all 18 to early 20s.
https://www.ncbi.nlm.nih.gov/pubmed/7983101The French women spanned 2.5 centuries and the mean was the year 1800. The average marriage age was 23.7 years and the average number of children was 6; 14 children was the highest number. The results showed that the French Canadian women bore their last child at an average age of 41.5 years compared to 39.5 years for the French women. The age at last birth declined over time in the French sample to below 38 years after 1840. The French Canadian sample showed 50% of women ending childbearing around the age of 40 years and 70% of women doing so in the 37-44 age span.
meanwhile puberty itself fell
5 years in the last century alone. from at least the 17th-early 20th centuries the date of puberty for girls hovered around 16-17 years. Some say the date was lower before that, but that seems speculative. As long as we've had good records, those are the figures.