Board Thread:The Last Sovereign Discussion/@comment-31763506-20170717113624/@comment-31763506-20170722060533

IIRC, the traditional age of majority of 21 in English speaking countries originated because physical development (in males) might extend that long, and was specifically put in place in response to heavy armor.

https://www.reddit.com/r/AskHistorians/comments/3mkri5/at_what_point_did_18_years_of_age_become_the/