What if...you do magic when you're 15?

[Katie]

No: HPFGUIDX 177422

So, a new question has popped into my head as I was rereading PS/SS for 
the fiftieth time...

What if, like Neville, you don't show any signs of being magical until 
you are older, but unlike Neville, you aren't eight...but eighteen. Or 
even just older than 11 - what happens then? Do you start Hogwarts as a 
15 year old in first year? Do you not get that chance, and you just 
have to learn magic on your own (which seems unbelievably dangerous)?

There was never an explanation for what would happen if your witchy 
side showed up late. This is really a nitpicky thing, i know, but I 
can't get it out of my head. There is such a big deal made of not using 
magic outside of Hogwarts, and making sure witxhes and wizards are 
properly trained and taught how to be responsible with magic...and 
never an explanation of what would happen if your magic showed up when 
you were 13, instead of younger than 11. 

Just curious...KATIE