ask to the child : 몇 살이야? (myut sal-i-ya?)
ask to the senior : 실례지만, 올해 연세가 어떻게 되세요? (sil-lye-ji-man, ol-hae yon-se-ga u-tu-ge doe-se-yo?)
ask to someone who is a few older than you : 실례지만, 올해 나이가 어떻게 되세요? (sil-lye-ji-man, ol-hae na-i-ga u-tu-ge doe-se-yo?)