At what age can you stop sterilizing bottles?

At 3 months. Sterilizing bottles is probably old school. In a clean household where bottles are washed in a dishwasher, most machines have a rinse cycle that is akin to sterile.
If all is very clean. A baby who lives in a clean environment, with clean parents, clean tap water, and a great sewage system, doesn't need to use sterilized water or bottles after the first 2-3 months. Sterilizing adds a tiny extra margin of safety, but is a lot of work. Many parents will use bottled water or still boil their water, in case the city water has some microbial spores, although this is rarely a problem.