I am massively judgy about adults in sports kit - I freely admit it - but have no issue with my young DC in it. They love sports, they play sports, and sports gear is comfy. Plus it's their choice, their clothing.
Obviously not for going out for dinner, or certain events - but for school, day-to-day, weekends, etc.
My (lovely) DM was very prescriptive about what I could wear and for the most part, I hated it. I just couldn't understand why I, the wearer, didn't get a say.