Hi there~ 살 and 세 both are same but those are used by age that how old than you. We use the 세 when we say someone's age of elder people and 살 is for oneself or friends,younger people. So, this is example. When you ask someone who is older than you: 나이(연세)가 어떻게 되세요? for younger people than you: 몇살이니? When you introduce to someone: 저는 00살입니다. for younger people that you: 나는 00살이야. As a result, 세 is only used when you ask and say someone's age older than you. So you can say your age below in korean. I am 20 years old. 나는 20(스무)살 입니다(이야/이다) Also, for using 세 : Only you have to use 세 for someone's age older than you or someone who you meet at first. How old are you? 연세가 어떻게 되세요. He is 70 years old. 그는 70(칠십)세 입니다.

살 is used with native korean numbers and is used in casual conversation. 세 is used with sino korean numbers in formal conversation or forms example: 25 years old can be: 스물다섯 살 or 이심오 세