You can say either........... "나이가 어떻게 되세요? (na-i-ga eo-tteoh-ge doe-se-yo?)" or "몇 살이세요? (myeot sal-i-se-yo?)" in an informal and polite way.^^' \^o^/

나이가 몇 살이에요? (informal, used to ask the younger or the same persons) 연세가 어떻게 되세요? (formal, used to ask the older persons)