The junior thing is difficult as they can see themselves as more capable than they actually are. We couldn't abandon a junior in trouble but equally couldn't leave one alone with a single adult. It can lead to difficult disruption of a ride and we have had occasions when a whole group has had to stop and wait for a parent to come and collect a junior in distress. The club ride rules make it clear EVERYONE must be capable of the ride standard, change a tube, have the route etc. We never leave anyone but if an adult chooses to ignore this that's their lookout. If an adult choses to peel off/tell the ride to leave them that is their responsibility.
Personally when I've been on a ride when a male has chosen to leave for some reason, tired, mechanical etc. I ignore this. However I've ended my ride and ridden back with female members on several occasions - even when they have initially protested - as I feel it is wrong to leave a woman alone in a rural area. Anything could happen, I'm not really thinking of cycling related. I've always received grateful thanks afterwards.