I was talking to another mother of a SN child. Her DS is 3 and can't get the free nappies until he is 4. She was angry about this but blamed it on the fact there are too many lazy parents not encouraging their children to potty train their NT children until they are over 3.
Now I actually think this is really unfair. I know loads of children who just weren't ready to potty train until they were 3.6 or over. One totally NT child I know was actually 3.10 when they potty trained.
I think if the child has a diagnosis or has been referred to a consultant who thinks there is something wrong but isn't sure what yet then you should be able to get the nappies from 2 or 3. Otherwise I think 4 is a fair age because you would get loads of NT children having free nappies just because they were later to potty train and that's not who it's there to help.