Brother’s age gap

My younger bro is 18 and dating a 31 year old he met at work. I think their relationship is cute and nobody in our social circle has problems with it but my mom told me in private she did not like him dating a, granny in her words, and she is to old. It got me thinking. I haven’t talked to anyone else about it and nobody besides my mom has complained so i assumed all was cool and his gf is really nice and chill, they seem to love one another. I don’t think 31 is super old to. What do Mangagoers think