It wont hurt them just as long as the nipple on it does not have nipple wroght in which case it needs to be thrown out. If you have a little one that sucks hard I would invest in the bigger nipples. Sometimes the smaller ones can only withstand so much then they break and become a choking hazard.
LOL I asked this same question! The only difference is the size in the nipple. If your baby prefers it thats fine. If they wont take it then you might have to try the 0-6 month one. My DD never cared and i thought the 6+ ones had cuter sayings so by 4months she was on those. :)