That's not a mean average, but one skewed by a high infant mortality rate - 30% dying in their first year. Those who survived infancy and especially childhood oft lived far longer (as in senior) years.
Further, given the level (as in lack of) affluence and sufficient/proper nutrition, people tended to be undernourished by today's standards, so young teens didn't look like teens at all. Heck, young teens didn't even look like today's teens till fairly recent decades.
Marriages then, as now, were prey much about economics and social connections. Not mature people looking to breed. Men tended to be older, as they had been more able to accumulate more wealth. Women tended to be girls.