My first truly psychedelic expereinces happened on weed. It's not psychedelic for me anymore, but it sure as fuck was when I first started using it.
I remember one of my first psychedelic weed trips quite well. I sneaked out of my house on a school night when I was 15 to go get high with my friends. We ended up getting sooooooo fucked up, and then while I was passed out in one of those good ol' dissociated psychedelic weed comas that are now a thing of the past for me, i had a "vision" of flying through the cosmos, but all matter on the period table had it's own color. And I flew past earth, and it was different from the rest of space because it was soooooo colorful, and it's because of the rich diversity of elements/colors that life is such a complex and interesting place in the universe.
It was pretty cool for a 15 year old who'd only smoked weed a couple of times.
That's why i listed weed & mushrooms though, cause weed isn't really a proper psychedelic, however, I did have my first truly mind expanding trips from smoking marijuana when I was a young teen.