The oldest student currently going to my school will be 23 years old 2 days before he graduates and the youngest is 12.
In the past I have been told that one student was 1 month shy of turning 25 when he graduated, he should have been in the guiness book of records.
Having a 22 year old student doesn’t surprise me having a 12 year old ninth grader does surprise me a lot because my school isn’t the type of school you go to if your skipping grades it’s the kind of school you go to if your skipping classes.
I know the 22 year old and I had to ask him why he would tell us that he was 22 because we all thought he was 17 or 18…if I was in my twenties and still in High School I wouldn’t be telling people.
It’s all good though he is cool people…anyway I just thought I’d share that because it’s very unusual. 
At the high school I went to they kicked you out if your 21st birthday was before graduation.
If you were really trying hard and wanted to stay and your birthday was only a couple months before graduation you could get a waiver but this was incredibly rare. More common was the people who had failed one or two grades before high school and then failed 9th grade twice and got the boot.
There was a 16 year old at my Jr. High School. He was a “code T” at the end of 8th grade, meaning he didn’t actually pass but they were “transferring” him to high school because if they failed him again he wouldn’t ever be eligible to even go to high school.
It’s not uncommon for schools which cater for alternative populations to be flexible enough to work with radical accelerants. They’re already thinking outside the square so they don’t assume that one size fits all.
I finished my Italian GCSE this year and joined the local secondary school’s Italian classes. it was quite cool getting to go back to school again 
And worryingly when I arrived no-one ever asked me who I was or if I should be on campus…