17 isn't too old. They switched me from PDDNOS to autism when I was 19. Of course the PDDNOS diagnosis had been just what they put on paper, while telling my parents I met the criteria for autism. (This was back in the nineties when to put autism down would have meant insurance wouldn't have funded treatment as autism meant "hopeless".) But that shouldn't even matter, lots of people can get their diagnoses changed much older than 17. I know someone who switched diagnoses from PDDNOS to AS in her twenties, and lots of people get diagnosed with any of the three (AS, autism, PDDNOS) up to their fifties and higher.
_________________
"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams