is 24 and 19 too large of an age gap for a relationship?

so uh, i've found out that my brother had been talking to a guy lately. I haven't learned that until they had been talking for a while, well, the guy that my brother was talking to was 19. he was 24. He seems to be incredibly mature for 19, i almost thought he was my age. and every now and then my brother always talks about him like " oh he really is amazing! " " Oh He's sweet and also an great listener plus, we also really had similar interests! " Im just a bit nervous about their age gap and was hoping for some advice on if I should continue to roll with this or not.