> Swimming lessons in the UK are mostly extracurricular, so they start whenever the parents feel like it. There are parent-and-baby swimming lessons for 3-month-olds, for those that want it.

Same here, but those parent-and-baby lessons are more about getting used to water, and not the structured lessons that lead to a diploma. And while regular lessons start when parents feel like it, nearly all parents feel like it between age 4 and 6. There's some discussion whether or not it's better to start early (I think it depends on the child), but starting at 7 would be considered late.

My son is 8, and, partially due to Covid lockdowns, partially due to a lack of talent, still doesn't have his A diploma, and he's the biggest kid of his group.