You realized that even though you are neighbors with Xavier for months... you realized you didn't know his real age. Even ask Jeremiah, Xavier didn't let him speak. Even judging by looks he can be either a teenager, young adult or maybe older?! Seeing you don't know his true age, you decided to do research... a.k.a stalking him.

"What are you doing?" Xavier ask after noticing you were spying on him.