Sports teams are classified as "Under 13" (or U13) for example. This would apparently mean that the players in the team are all under 13. In practice, it is usual for it to be interpreted as "Under 13 on the first day of the school year (September 1st)"
For football (for example) a team of "under 13s" could have players who were aged either 12 on September 1st of that football season. This means that players don't have to move teams on their birthday, but play with others from the same school year.
The naming varies from sport to sport. I football, the adult men's team is doesn't have any qualifier (eg "Chelsea"), but if clarification is needed you could say "the senior team". The adult women's team could be "Chelsea women", although many women's teams are dropping the qualifier as demeaning. Then there could be a youth team (U21 or U18 are possible) and then separate teams for U16, U15 and so on down to as low as U5.
Mark plays for Uxbridge Under 18s football team, but next year he is hoping to play in the senior team. His younger brother plays for the U14s. His dad coaches the U12 girls, and plays for the Uxbridge veterans.
The youth teams in Uxbridge have been very successful this year with the U16 winning the county cup and the Under 8s coming second in the 5-a-side league.