A lot of signs are most obvious when you are a child/teenager.
Also, I saw a video about autism and I totally saw myself in what was described.
For example, hypersensibility to external stimulus is something very known, but did you hear about "hyposensibility to external stimulus"? I, for exemple, could go outside wearing only a t-shirt and pants during cold days of winter. Yes, hyposensibility to cold, for exemple, is a symptom too.
They were also talking about how boys and girls were not diagnosed the same way. If you look at statistics, people who are diagnosed of autism are mostly boys, and that is in part due to society.
For example, a boy who has his bedroom full of star wars stuff (figurine, poster, bedsheet, etc), you may think he is autistic.
However, a girl who has her bedroom full of unicorn or hello kitty stuff, you may think "this is just a girl being girly".
But they are both the same thing (having very specific interest and being "borderline" obsessed with it)
A boy who is very shy and who doesn't approach people/doesn't get approached easily, it's autistic
A girl who is very shy and who doesn't approach people/doesn't get approached easily, it's cute
That's how many girl were not diagnosed