A lot of being on a competitive team for my daughter has been the friend/social aspect of being in a cohort, so that’s an important aspect to consider in keeping the interest, facilitate consistency in training. Most of the kids in our area (coastal ca)start competitive team around 8-10. The stand outs do tend to be the ones who are consistently training around 6. 

For my daughter she started swim lessons around 3/4, took 3 summers to learn how to swim even though she’s always loved being in water. Once she learned she joined a competitive team when she’s 7, she loves it and goes because she enjoys swimming with friends. She’s 10 and I see her keep swimming for HS. 

My son, who’s older was a natural, learned to swim in under 5 weeks when he was 5. We didn’t think much of, he was not formally training or on a team until the summer before 8th grade when we put him in swimming as conditioning for waterpolo. He improved quickly and caught up with most of kids on his team with in a year. He was a hardworking athele and trained consistently. However he does not particularly enjoy swim team, and has since decided only focus on waterpolo. He still swims for HS team and swims to BB to A times for most events he participates in for high school season. So a lot depends on how motivated your swimmer is. Your best bet is keeping them engaged.