Dunno if picked on is the right phrase but...
Yesterday was at the park with ds 4.5, he approached two older boys - around 9 I'd say - and began talking to them about toy story.
They made it clear without being too rude that they weren't interseted in talking to him.
I tried to distract ds but he was adamant that he was playing with his "little friends".
Now, he has a tendency to repeat himself over and over so I can see - quite apart from the age gap - why the boys wouldn't have wanted to talk to him but he is also the sweetest little thing, very gentle and warm and genuinely felt these boys were friends 
Fairly quickly one of the boys was very rude to ds; "just go away you're boring us" "stop saying the same rubbish over and over and over"
At this point I said he's only 4, he doesn't mean to be annoying he's just trying to be friendly. I then told ds we were going to the sand pit - and even though at some level he knew the boys didn't like him (he was clearly a little bit unsettled by now) he said that he just needed to say goodbye to his friends first He did, and was ignored by one and told "go on then" by the other.
I have to say that the vast majority of older kids - he ALWAYS seeks out the older boys to play with - are absolutely lovely with him and happy to include him, but the odd ones like these two are quite mean and usually pick up on his repetative speech.
On the one hand I want to whisk him away from kids like that but on the other I want him to understand A)that some kids just wont want to play with him and to accept that and B)that he has to try harder to have proper conversations with people - give and take, ask questions, listen etc....
So is it better to just stand back and let him learn the hard way?
(I'm inclined to think not but there was a period where his little school pal across the road didnt enjoy playing with him because ds was too repetative - in speech and games etc - and after a week or so ds did adapt his behaviour and they are firm friends again, so....
)