You don't mention the age of the guy, but if he's older than you it's evident why he said it and what he implied: you're too young to his taste. Don't worry, it's a temporary and self solving problem: you'll grow older :D
Heck... I don't know. I occasionally ask that to my guy friends when I find out they have a sister just to jab 'em a little. You know. Tick em off. Tease them. I don't think I ever asked that to a girl I knew.