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.