Salaam. I think it's all personal preference. But most importantly, as long they cover their awrah, i don't think we should judge them. Let Allah judge them. Some people don't do it on purpose, they could be from a football game or other activities and maybe they forgot to bring a more appropriate clothing to prayer. Happens to me at times too. Infact we should be proud youngsters are making the effort to go to mosques to do prayers!