I found out when I was 29, diagnosed at 30.
I spent my entire life trying to figure out why everything was so difficult. Why I felt so fundamentally disabled but I couldn't explain how or why.
I struggled so much in teen years but I couldn't figure it out - and doctors just though I was lazy or making excuses.
The same thing happened during my 20's, except autistic burnout got worse and worse over time. Due to this repeated burnout for over a decade, by age 28/29 I was experiencing intensely worsening autistic traits.
Around the same time I met a fellow trans friend who has a lot of similar life experiences to me. She thought she was autistic, and it clicked that I might be too.
A lot of research later... I knew I was without a doubt. I chose to get diagnosed because I want to recover from autistic burnout and try to live the life I never got to have.