Young people did tend to be what we would consider old for their ages in the 15th. Century world of the Wars of the Roses, which is the background for Westeros. Edward IV was 18 when he fought his way to the throne of England. His brother, Richard of Gloucester, was charged with commanding a wing of Edward's army when he was even younger. Child marriage was common. Edward's younger son was married when he was 5; his bride was 8.
Ned and Robert seem to have been about 18 when Robert wins his throne, so that they fit very well into the prototype.