to comment such as “b) mentally and physically boys aren’t ready at 18.
Physically not ready? What’s the major physical difference between an 18 year old and a 23 year old? Emotionally and economically you’d be right.”
I would aruge that people are “”ready” to get married (and become parents) until they are in late middle age, whether from an emotional, economic or mental state — but since that would lead to extinction HaShem biologically programmed people to feel an urgent need to get married when they are young enough to have children. Since HaShem designed people to get married before they are emotionally, economicly or mentally ready, this is obvious a “feature” and it isn’t for us to speculate on “why.”