Sadly, while parents often report noticing differences in their children at a young age, many children are not diagnosed for years. In cases of milder symptoms, Autism Spectrum Disorder can go undiagnosed into adulthood.
I did not receive a formal diagnosis until the age of 42. This was despite me spending most of my 17 year teaching career as an ASD specialist teacher.
It was a change in career from (the highly structured) teaching to self employed that things started to really unravel and become a lot more obvious.
I struggled through the previous 41 years of my life not quite being able to pinpoint what I was struggling with and why.