ELI5: Why do relatives or my parents friends children who I used to be close with stop wanting to talk to me now I’m older?

I’m under the impression this occurs with the vast majority of people. Just to add, I see myself as a pretty outgoing and friendly guy.

Essentially I used to be very close with my cousins, always playing together, watching movies, YouTube playing video games. Now they don’t really wanna see me when they have the chance or they don’t speak to me at all when we are at family gatherings. Same with my parents friends children, we used to be incredibly close, we practically grew up together as friends, however at gatherings we barely say words to each other now. What happened? I try to engage with them but they don’t seem to be interested. Is this just what happens when we grow up? Do they just not like me anymore?