You know, Koreans generally don't use polite from to someone younger if they are already very close. But let's assume they are not familiar with each other yet and use the polite form. There are a few choices for "You are younger than me". 1) 저보다 손아래시네요. (손아래: junior, 손위: senior) 2) 저보다 어리시네요. (어리다: young; under age) 3) 저보다 젊으시네요. (젊다: young) I would prefer in the above order. 3) can sometimes mean "look younger" rather than "at younger age".

you are younger than me