Why did nobody notice?

I was diagnosed last September aged 23. I have had mental health problems for over 10years though. Been in hospital dozens of times. But what i want to know is why didn't anybody notice my Autism as i was growing up? Teachers and Doctors and dance teachers etc. I've been to 4 schools - infant, junior, high school and grammar, and at both secondary schools i had problems yet nobody ever questioned why. I feel really upset, angry and let down. I know i am a moaner but a bit confused and cross. Mum feels the same. Is it worth writing a letter to my schools?

Anne