round eyeglasses are perfect for people who have square or angular faces because round glasses can help minimize angles of your face. So people who have square or rectangular faces can go well with round glasses.
Yes, John Lennon's round style glasses looks cool. And this is not a matter of age. So, it won't be weird for a 18-year old girls. As long as the round eyeglasses flatter you, you will look great with them.