Most start skating with their coach and the age gap can only get smaller from there.
I do not remember details, but I there was a girl who asked her coach to take her through her senior pairs test. Afterwards, she asked him to skate at sectionals with her. Since this is pairs, they got a bye and skated Senior at Nationals, where they did pretty well. The girl was 16 at nationals and the coach was more than twice that. Another coach was hired to coach the team. I'm not sure if I would call them "competitive" because as far as I know they were just doing it for fun, not to qualify for anything, but they skated at nationals.
Size difference is more important in pairs than dance because of the twists and throws. In dance, looking similar can be helpful (see Gamelin and Gamelin, twins who compete senior dance). However, I think good technique and having fun are more important than what your partner looks like.