https://pmc.ncbi.nlm.nih.gov/articles/PMC10357465/
In some children, autism symptoms can change over time, getting more or less severe, and sometimes this will shift them into and out of diagnostic criteria, especially as there is evidence diagnoses needs fewer criteria now.
It's also the case that often transitions, say to secondary, or to university, are a point for diagnosis as the child or teen is confronted by new problems they are not equipped to deal with and this highlights them.
Diagnosis is also a matter of political and economic factors, I knew my dd was severely dyslexic but the school insisted she was just not a great speller and it took til after lockdown and me paying for it myself to get it on paper, and now no-one disputes it at all as it's so obvious.
One of my children, now an adult, chooses not to go for diagnosis, and has done since about age 15, and I can't make them. They don't want to and say there's no medication they can take anyway.
At the more severe end, this may be a very odd thing to do, but in the grey areas I think it's quite explicable why people wait, see if things improve, act when it all goes wrong, and fail to use the systems which are not set up for quick diagnosis at all.