When indicating how old a person is, you should use pure Korean numbers along with the word "살" which is a counter for ages. For example:
저는 열 살이에요 - I am ten years old
저의 여자 친구는 스물여섯 살이에요 - My girlfriend is twenty six years oldIt would sound weird to use the Sino-Korean numbers to indicate the age of somebody who is under thirty. However, after thirty, it is not uncommon to use the Sino-Korean numbers instead of the pure Korean numbers. The older the age, the more likely you will hear the Sino-Korean number used instead of the pure Korean number. After 50, you are most likely to exclusively hear the Sino-Korean numbers.
You haven't learned how to ask questions yet, so it is difficult for me to explain how to ask about a person's age. Nonetheless, it would be good to memorize this sentence because of how common of a question it is:
몇 살이에요? - How old are you?

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LEARN KOREAN (한국어) [GRAMMAR] #1
RandomThis is book #1 for the Korean Language series. Try to understand all of the notes in this book before you proceed to the next one. Good luck ! This book contains Korean grammar. Try to understand it so that you can understand how to make a sentence...