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!
Official pronoun for the mysterious Satoshi Nakamoto the inventor of Bitcoin who could be male, female or a group of people including both genders.
“He went by the name Satoshi Nakamoto, but nobody knows who he is.”
“So if nobody knows, Satoshi could be a she, or even a they!”
“Fine, nobody knows who t/s/he/y is… don’t be so woke”
a plain of land that looks like it would be mars, but it isn't
"Look at how Mars-y my backyard is"
"hey is that a 12 y/o child doing 12 y/o things in brookhaven?" "yes :|"