y/a is "Your age". I have been trying to search it but no one defined it 😅. Usually, a writer of fanfiction sometimes includes the reader in the story, making it second-person POV. Here's some examples:
Person 1: Hey y/n, you're y/a years old right?
Y/N (Y/N stand for "Your Name", also is usually used by a writer of fanfiction): Yes!
M.I.A.'s response to Y.O.L.O., based on her Eastern cultural heritage. In Hinduism it is believed that people are continuously reincarnated, so it's acceptable to do somethingmore than once.
Also a title of a song on her 2013 album Matangi.
"Why does everyone keep saying Y.O.L.O. and then doing the same stupid stuff? It's such a waste."
"Yeah, back where I come from we say Y.A.L.A. before we do stupid stuff cuz we know we're not wasting any time in this life. But then they invented karma to make us feel bad."
If you want to tell a friend to watch a video or listen to a song via 'youtube', then instead of asking them to do it A.S.A.P or go-and-watch-that-video-now, you simply say Y.A.S.A.P (youtube that A.S.A.P).