I had protein in my urine when I was much younger, in my early to mid twenties. After loads of investigations, including a particularly enjoyable 24 hours when I had to carry a huge sloshing container of urine around and pee in it, it was concluded that there was nothing wrong with me and I just had leaky kidneys. So it might be nothing worth worrying about. Apparently it's quite common in younger people who are very active.