Playing sport for your country?

most people play sport these days just to keep fit and healthy,and also because its fun...sometimes people get incredibly good at sport and excel...eventually they may play for their city,state, country etc...how good do you have to be to play representative sport for your city/country...do you have to have natural talent and get into under 15 sides or under 17 sides etc early on in your career or can you wait and become good at sport later in life like early 20's???? If your average at sport during your teens there is no point aspiring to represent your country later in life is there??? Or can you go from average to great into a short space of time?
where do young children get all their motivation from...most will never actually get into representative teams yet they still devote so much time to sport....