Is it weird I don’t like my brother’s relationship ?

So I, F26 have been close with my brother forever, he's amazing and I hope he finds happiness, but he introduced me to his boyfriend a few days ago, don't get me wrong, he's the sweetest little guy, my brother is in his late thirties (37) while his boyfriend is 20, like he JUST turned 20, am I wrong for being weirded out about it? He says I'm stereotyping and I'm just genuinely confused why he's almost 40 dating someone who was in high school a year ago(technically around 8 months ago).