I can confirm this is by design, esports and soccer are only for Sims attending University.
I have a teen enrolled in University, and I cannot join soccer team. I had him graduate high school early.
There is definitely a bug of some sort though. You can graduate early as a teen with the high school pack, and enroll in university. You can apply for scholarships. Including the athletic one which requires you to be a member of the soccer team. My teen did this, and joined the soccer team, but after day one of uni and soccer practice, soccer team has disappeared from the career panel and she's lost that scholarship. She can't join the soccer team through any of the usual routes that a young adult would have. So, if teens aren't supposed to join the soccer team, they shouldn't be eligible for the scholarship, therefore there is a bug.
