Why are like some sophomores in high school already 17?

im a senior in high schoolll and i was born 1993. I knoww sophomores that were born in 1994 and are already 17!!! I was 15 when i was a soph. Wtf is going on here. Someone told me that its becasue the started schoool later ionooo. I feel like im graduating really young thooo. I even kno a soph who was bron in 1993 older than me by a couple months!! Someone explain this too meee !!