The basis is called judgement. Would you allow a six month old female child to have sexual intercourse with an 18 year old man? There obviously has to be a cut off at some point; so why not 21?
Judgment is based on criteria. It does not stand alone as you appear to have it standing. Enumerate the criteria, and the reasons.
English law pretty much echoed the practice of the biblical era until perhaps a couple of centuries ago. The idea was to target puberty, the point at which the individual involved could understand sex.
Why take away the power of parents to grant the hand of, say, a 16 year old daughter to some other parents’ 16 year old son, when he is set up to be able to support her? I could as easily ask that.