Whenever they'll take it!
I'd say they're large enough to try. Start out with cyclopeze and only small amounts when feeding normally. You can also try finely chopped mysis and shaved squid. However, once you do, start keeping really close eye on the water quality, so to notice any ammonia spikes from uneaten food.
You may also want to try growing out some brine shrimp to 1/4 size, and gutloading them well. Some of the larger ones I bet bbs looks too small to them right now.