Japanese people and their birth year

I don’t know if this is too weird to ask but I basically know no Japanese person to ask lol, or even someone that knows about this.

So basically me and my brother got into a manga these couple of weeks, and I opened twitter to keep up with updates, and mostly because of amazing fan art. I noticed that most of the fandom for this manga (that i encounter, at least) is Japanese people, and in their bios they have (for example): 98(99). I didnt think anything of this until i encountered even MORE japanese accounts with of these, ppl with 95(96), 97(98) etc. I think this has something to do with birth years? But why do they add a year to their birth year? (Or is this nothing birth year related??) Idk it just seems like an interesting fact to me and I want to know what it is

Does anyone know anything of this?? Is this a Japanese thing?? I honestly have no idea who else to ask, or if this is even a Japanese thing or another thing altogether. So sorry if this ends up not being Japanese related