I did a bit of research on this and you can stop sterilising once the child is mobile and picking stuff up off the floor, etc. However, you should always sterlise bottles (and more specifically teats).
As soon as they started crawling.
Same here...as soon as they got moving and I realised I had very little control over the icky stuff they tried to eat. ;) As long as the bottles/cups were 'clean', I stopped using the steriliser.
About 9 months... I dont think theres any hard and fast rule but when they are putting all sorts of things in their mouths that are unsterilised, it may be the right time to stop?