Age can pose problems but you will find a number of folk on WP who got a diagnosis later in life.
I did not have any suspicion that I might be Autistic until I was 64, and I'd never even heard of the Autism Spectrum. I got my diagnosis shortly before my 65th birthday.
Whoever does your assessment might want information about your earlier years. I had a lot of records for the Psychologist to use, my bride met with the Psychologist, as well, and my Dad is still alive and we got a little bit of information from him. (I don't know which specific records interested my Psychologist, but she did mention reviewing the journal my long-deceased Mom had made of my first year of life.)
I sort of enjoyed the assessment process. It was kind of fun. But parts of the assessment process were obviously designed for young patients--those parts of the process were funny!
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When diagnosed I bought champagne!
I finally knew why people were strange.