This is just a general question really. Ds1 is just a few weeks away from his 6th birthday and his behaviour is starting to change. To quote ds1's teacher "I don't know if it's an age thing, the changes at home, or if I've just had an easy time of it so far."
Things started to go downhill just before the Easter holidays. He seemed to change into a sulky teenager overnight. He is usually quite passive although he will stand up for himself if people try to push him too far. At school towards the end of term he was becoming upset almost constantly. I put this down to a mixture of tiredness because of the end of term, the disruption to his routine with the arrival of dd, and the virus which he and ds2 both came down with during the last week of term.
The first week of the holidays were a living hell with ds1 and ds2 either attacking each other or having meltdowns. This seemed to be illness-related because when I managed to sneak some paracetomol into their drinks their behaviour improved drastically.
I'd expected some kind of reaction to dd's arrival but tbh they can't seem to get enough of her. Ds1 makes a beeline for her whenever possible and will chatter away to her for ages. He's always telling people (unprompted) that he loves his sister and that he thinks she's very cute etc.
By the end of the holidays both boys were back to their usual selves and I hoped that getting back to their usual routines would help. Ds2(3yrs) bounced back as soon as pre-school started. Ds1 was very happy about being back at school but his teacher has told me that he is starting to refuse to do things - probably quite normal for an average child of his age but unheard of for ds1 since he's been in Yr1. Lots of "I hate doing this!" and sitting with his hands clamped over his ears.
I can't for the life of me work out whether this is related to his ASD and sensory problems or if it's perhaps related to his age. IIRC a few MNers have had problems when thier ds/dd has got to this age but using the MN archives is difficult as I can't seem to get the search facility to work properly. 
So, if you've made it to the end of this post, did you notice any sudden changes at this age? Is this just ds1 being ds1 or a case of the Sodding-Sixes? :)