I'm aghast that people are actually dictating to teenage boys that they can't wear sportswear if they choose to! Do people really micromanage their teenagers to this degree? How bizarre.
I have 14 and 16yo boys. One wears button shirts, suit jackets and smart shoes by choice (and sometimes a fedora and a greatcoat too). The other lives in Adidas, Nike and Man Utd kit, has more shoes than Imelda Marcos (he loves poncy trainers and will save up for them) and spends far too much time on his hair.
They're young people, forging their own identities and expressing themselves - it's important! The idea that I would choose their clothes, or tell them "you're not going out in a football top, go and put a shirt on" is bloody laughable.