I have continued to read the book on Bonhoeffer. While in his mid to late 30s he gets engaged to an 18 year old girl. Back when I actually contemplated getting remarried, I never seriously considered looking at someone that young because I found it unlikely that someone so young would be mature.
Parents of course could raise someone to be mature but lack of effort and societal pressures keep the majority of people from doing what needs to be done.