In many places Loki is described as Thor's youngest brother. However, in Thor:Ragnarok, Thor is talking about an incident from their childhood
This one time when we were children he transformed himself into a snake and he knows I love snakes so I went to pick up the snake to admire it and he transformed back into himself and was like “YARGH IT’S ME!” and then he stabbed me. We were eight at the time.
So if they were both eight, then they should be the same age.