This is a toughie, everyone has their own ideas about potty training - for kids with or without autism.
All I can say is what I have found.
Eldest child (not autistic) battled,ignored everybodies advice about "just keep trying" ..and when she was 3 and a bit she was ready and learnt in a week!
Youngest child (autistic) had no idea, didn't worry about it, at 5 and a half he potty trained - no battle, night time too. BUT I never thought he would be potty trained. I thought he would be in nappies for life.
So, from my experience I would say - don't battle and make life HORRID and STRESSFUL (lets face it , it is stressfull enough), wait until the child is ready.
Does it matter if she is 7....no. 8...no.......
As I said - everyone has their own ideas, mine might be directly opposite to someone elses.
GOOD LUCK!