Thanks for all your tips and suggestions. The record of Andrew jr's birth has the names of both parents, which suggests that Andrew sr 'stood by' her and accounts for Andrew jr taking the Boyd name. As Annie 65115 points out, there was no legal reason to prevent a marriage at her age, but why wait 7 years after your first child is born to get married?
Unfortunately, I haven't been able to trace a record of Andrew sr's birth yet, at least not in the local area, so I don't know his age when Andrew jr was conceived. Similarly, I haven't been able to find death records for Andrew sr or Janet.
I've tried some variations on both names, but nothing conclusive has emerged; I've restricted myself to Paisley so far, but I acknowledge it makes sense to search a little further afield. An older Janet could well have come from somewhere else at some stage, but the marriage record says 'both in this parish', which is all I have to suggest that she was local.
Janet's parents were (I think) John Bryson and Janet Thomson; my Janet had 6 children - the boys were Andrew and William, and the oldest girl was (yet another) Janet.So, the name Janet was passed on to the next generation, but not John.
Once again, thanks for your interest.
Eddie