While all manufacturers seem to size their baby clothes differently, one thing is consistent across the board. If there is a single age listed on the tag, that is the maximum age the manufacturer expects your child to fit into the outfit.
Unfortunately, that doesn't always mean a lot. Since all babies grow at different rates, it's not uncommon for a 12 month outfit to stop fitting at 6 months; or, some babies may be able to wear a 12 month outfit well into their 16th month.
Generally, it is safe to assume that if a size label reads "12 months," you can interpret that to be 9-12 months. The same holds true with clothes marked "18 months." Plan on those outfits fitting from 12-18 months of age.
It's also not a bad idea to tuck a baby clothes size chart that also includes weights into your wallet; when in doubt, you always have it as a reference.