Radak: It is not because he was a youth, rather, due to his nice appearance. He thought that he cannot be a warrior, for a warrior's nice appearance will not last, due to exertion and toil, and he is outside at the time of war in freezing cold, rain and great heat.

Malbim: He was a youth, and his nice appearance shows that he is delicate and pampered and not experienced in harsh wars, only his redness due to hot blood lifts him to what is above his nature.