In my early days of collecting, things were more neatly wrapped--ie: most dolls prior to the composition era were considered antique. Now, of course, that is changing--within a decade or two, the vast majority of composiiton dolls will be antique dolls. I'm not sure where the general rule that an antique doll is 100 years old or more has come from, but the U.S. Customs office defines antiques as objects over 100 years old, so it is possible that their definition has influenced our hobby.
Currently, dolls that are antique include most French and German bisque dolls, most china dolls, and early wood, wax and papier mache dolls. Some early composition dolls, as mentioned above, are now considered antique. Eventually, all dolls will be antique--in 2059, for instance, the first Barbie dolls will be antique dolls! Shirley Temple composition dolls will become antique in the 2030s.
Some collectors prefer to simply consider all French and German bisque dolls to be antiques--even those made up until the 1930s. There is some sense to this, since, for instance, Armand Marseille, a German doll manufacturer, used the same 390 mold from the late 1890s through the 1930s. Other people like to consider all composition dolls as vintage and not antique. Obviously, this system of classification would also eventually change over time.
Older dolls that are not officially antique yet are generally referred to as vintage dolls. Some refer to them as collectible dolls (although antique dolls are also collectible, so I think that can become confusing). The vast majority of composition dolls, hard plastic dolls, and even now some vinyl dolls are considered vintage.
When purchasing antique dolls, in pays to be educated and to buy from reputable antique doll dealers; antique dolls of high value have been known to be faked or doctored or have undisclosed repairs. It pays to be cautious and knowledgable, especially when starting out as an antique doll collector.