Can you be best friends with someone that's twenty-three years older than you?

I'm a fifteen year old girl with cerebral palsy and one of my assistants at school who is in the SPED department at my school and I have gotten really close. We tell each other almost anything (like how we both had to have surgery and get a brazillian from our surgeons or about how she was in a mental institution for trying to kill herself when she was fifteen.) And we can swear around each other and we laugh all the time. The thing is, she's 37 and she's very buddy-buddy with a lot of the other kids that she works with or used to work with, but I don't know the full extent of their relationship. I'm not jealous at all, it's just I don't know if she is or not or if that would weird her out or not, but we laugh all the time, trip over our own feet and say, "I love you!", we could talk for hours on end, she defended me against a girl I didn't like, we have the EXACT same view on all of our mutual "kinsman", she told me when she lost her virginity, she told me she had shooting pain in her butt, we have inside jokes about my RR teacher, she bought me a birthday present, I showed her some of my really intense, profound poetry. We've told each other secrets, she could tell when I was upset when everyone else couldn't. Does that not say best friend to you? And if so, should I tell her. I was planning on telling her the day we go off-island for the Special Olympics of Massachusetts (SOMA) Cape Cod School Day Games, which is a sporting event type thingy with my Adaptive PE class and the teachers that are in APE with us, because it is supposed to be one of the best days of your life I heard from EVERYONE who went last year and I want to find a really special moment to tell her. And then, before she leaves for her surgery in June, I wanna give her a homemade card. Any advice?