I asked about it at our last hospital review meeting in November, to help with J's anger. J was just approaching 8 at the time and I was told that they wouldn't even consider prescribing it for one so young (they actually thought he was 9), though I have heard of it being given to much younger children. In the US they seem to dole it out willy-nilly, but I think medication is more of the treatment culture there.
I guess it would depend on the level and type of behaviour that needs to be addressed, whether or not environmental changes have been implemented and had any effect, the age of the child and the level of disruption the current difficulties are causing to their life. I'd expect the opinion of the consultant would matter a lot too - some have rather narrow thinking and stick very rigidly to 'the rules' so may not prescribe on those grounds.
Essentially Rispiridone is an anti-psychotic drug and it's not recommended for very young children, but the fact that there seem to be quite a few children well under 15 taking it shows that there is room for manoeuvre in that.