I appreciate that they most likely didn't grow up in a time and place where diversity was common and certainly not one where it was celebrated
Well there you are then. They comment on the relatively recent levels of diversity because to them it seems like it's happened overnight. I've noticed a huge difference compared to how it was in my childhood and right up to my thirties, and I'm not anywhere near my sixties yet so if it's a source of amazement and fascination to me, I imagine it's off the scale for them.
AIBU to be surprised that a more modern view point hasn't rubbed off on them?
You said yourself they didn't seem judgmental or negative. I'd take that as evidence that a modern viewpoint has rubbed off on them to some degree.
Also AIBU to think that if they are thinking these things, they shouldn't be saying them out loud?
Maybe not. It would might be advisable to be a bit more subtle about it. Especially when there are smug, condescending, ears-on-elastic people just willing them to trip up and confirm themselves as bigoted, racist, old person Brexiteers.
or hypothesising on what medical condition might be behind mobility or speech issues.
When those volunteers were at school many of those children with disabilities or additional needs would not even have survived infancy, never mind been in mainstream education. Inclusion now is a relatively recent thing.
I think there has been such a boom in the numbers of children in mainstream school with developmental delays, possibly due to being born very prematurely and surviving, or being born with genetic conditions and syndromes that can be treated to have much better survival rates now whereas two or three generations ago they would not have made it that far.
There is a huge rise in the number of children (and adults for that matter) being diagnosed with ASD and ADHD, and such a huge drive to create awareness and acceptance of it that you can hardly blame them for being interested and attuned. That's the thing about awareness campaigns. They make people more aware.
It's only natural that people will want to show that they are enlightened and sensitive to the signs and the needs of children with various conditions, and the possible reasons for them. If they are doing that rather than just putting everyone into those old school boxes of spastic, retarded, mongol, backward, naughty, weird etc., that were used when they were growing up then they very much HAVE allowed a modern viewpoint to rub off on them.
Hypothesising on someone's medical condition just seems intrusive. Take them at face value/deal with the issues they have. What does it matter what actual label they have?
I have a sneaking suspicion that you trying to tell us that they speculate about children from Muslim families having disabilities caused by genetic defects as a result of several generations first cousin marriage. Am I right?
Finally AIBU to be petrified of turning into one of them in 30 or 40 years time, possibly not in relation to the same issues, but something new?
It will happen as surely as the earth keeps turning. There is little point in being petrified about the inevitable. When it's your turn to do it you won't realise or accept that you are doing it, but the younger people around you will and they'll be rolling their eyes and starting threads about you.