We switched some of her nipples at 3.5 months (we use Playtex Nursers). We found the silicone slow flows were making her frustrated. A 5 oz bottle took well over 30 minutes, and she spent half the time popping off and taking in air and hollering about it. We went to fast flow silicone NaturaLatch nipples and fast flow Natural Action nipples (these are our favorites). For the latex NaturaLatch nipples, we slightly enlarged the holes on the slow flows, because she was frustrated but the fast flow variety made her choke. She still dribbles occasionally, but we have so much less gas and spitting up now at 5 months. Every once in a while we accidentally use a slow flow, and she lets us know. Your little guy will let you know too. If he likes drinking from the slow flow, don't change it. But it is okay to change it when he's ready - fast flow isn't always bad, especially if he is happier with it!