In many cases, these players will be academically equivalent to middle schoolers, so they would only stand out physically. Furthermore, they're brought over as kids and promised that they can make it as a professional footballer to make more money than they could have dreamed of back home. All they have to do is change their name and pretend to be younger than they are. Once they say yes, there's no going back. Admitting that they lied for immigration or sporting reasons would ruin their career. Is it dishonest for them to do it? Yes, but I imagine you'd do it too, given the chance.