Prof Sally Holland said they were not getting the support they were entitled to by law.
Lucy Williams, from Wrexham, has Asperger’s and said for years she thought “there was something quite weird about me”.
The Welsh Government said it had announced it was improving services.
The Welsh Local Government Association (WLGA), which represents councils, has been asked to comment.
Prof Holland’s report looked at the experiences of young people with learning disabilities as they become adults.