ADHD was first recognised as a condition (but without that name) in 1902. It first appeared in a manual detailing mental disorders under the name of 'hyperkinetic impulse disorder' in 1968. It became 'ADD' ('attention deficit disorder') in a new edition of that manual in 1980, with the 'H' (for 'hyperactivity') being added in some, but not all cases of ADD.
Source:
https://www.healthline.com/health/adhd/history
We were referring pupils to the school psychological service, in cases of suspected ADHD, when I was teaching back in the 1980s.
The term 'autism' was coined in 1911 but it wasn't until the 1940s when it came to be used to refer to the emotional and social development of children. However at that time it was (wrongly) linked to schizophrenia and it wasn't until the 1960s before it was recognised as a condition in its own right.
Source:
https://www.webmd.com/brain/autism/what-does-autism-mean
However at that time the prevalence of autism wasn't fully recognised, with the condition being regarded as rare. These days, charities such as the National Autistic Society cite 'more than one in a hundred people' as being a reasonable estimate of the number of people who're on the autism spectrum but many people believe that the figure could be much higher (possibly at around 5% of the population).
Trying to work out how many people are on the autism spectrum is complicated by the lack of clear definitions of exactly what 'autism' is. Some experts take the view that
everyone is on the autism spectrum in some way or other, with some people just being a bit further along it than others.