“Yet that’s exactly how you’re using the statistics you’re citing for time periods when the average marriage age was 20.”
Which time periods are those when the average marriage age was 20?
According to the table you posted, 1650 and 1950. Needless to say, geographical variations within any given time period are also very large. In the US Census data for 2000-2003 broken out by state, there’s a range of over 8 years for the different state averages.