DD was diagnosed at 17. She was v high functioning and well behaved so never picked up. The diagnosis was a lightbulb moment that put in place a history of absolute brilliance tempered by two steps backwards and never fulfilling her potential which we, her parents, pit down to a lack of application.
At 15 anxiety, depression and self harm reared their heads and she had to repeat yr12 due to being unwell. These are often comorbidities in ADD when itnis undiagnosed.
Getting help is very difficult. CAMHS were not interested in looking at a neuro-developmental disability or finding a cause for her anxiety and depression; only for problems with the family and that was open and shut with no helpful intervention offered even though her self harm was also manifesting as small overdoses.
We had to find her support privately through a psychiatrist specialising in young people. It was the psychiatrist who assessed for ADHD/ADD and ASD (although that was not diagnosed ASD). The assessment came after therapy and anti-depressants which helped but didn't resolve.
The diagnosis was dd's turning point and recovery commenced thereafter. She takes ritalin when required. She is going to Cambridge in September.
I urge you to follow this up before the teenage years but suspect you may have to seek assessment privately because ime CAMHS really don't want to know and the community teams do not have the up to date skills to know when an assessment may be required and they do not provide enough support for the underlying issues in any event.
It varies though around the country. We live in Surrey but your services may be significantly better than here. It woukd be hard for them to be worse to be honest.