I haven't met a 25 year old yet that didn't think they know everything, myself included--14 years ago. I admit it. I'm ageist.
Whatever the fuck that is.
How do you know they thought they knew everything? Either you're a mind reader

, or there is at least the
possibility that it's your own ability to make accurate perceptions of people that's faulty.
For two reasons. One, I have a 25 year old friend, and both he and girlfriend, aged 22 both come off this way.
And two: I remember that I was also exactly like that when I was 25. So was my brother.
Maybe it's not the same for everybody, but it's definately been so in my experience.