What is an antique anyway? Most people agree that antiques are old-100 years old or more is a good rule of thumb. But that doesn't mean that everything old is an antique. An antique should be "authentic." That is, it should come from a time and place where such things were in style. And it should have been constructed with the tools and methods that would have been used to make such a piece.
In general, an antique should be handmade. Certainly, many lovely pieces of furniture have been machine-made. But anything made by a machine was probably made multiple times. In other words, the piece isn't unique. A real antique should be one-of-a-kind. That doesn't mean a craftsman can't have made several dressers or chairs or whatever in a similar style. But each one will have its own character because they were made one by one, by hand.
As much as possible, an antique should be in the original condition in which it was made. If it has been altered substantially, it ceases to be an antique. If a piece has been refinished, resilvered or regilded, reupholstered, or otherwise repaired, it loses some of its antique value. If the knobs on the drawers or the feet on the chair aren't original, the piece is a little less of an antique.
How much of that is permissible before the object ceases to be antique at all? There's no firm answer to that. There's an old joke about a man boasting that he has his great grandfather's axe. Of course, the axe head has been replaced twice and the handle has been replaced three times. The axe is quite functional, of course. But it's no longer an antique. Just realize that the more alterations that have been done to a piece, the less credible is the claim that it's an antique. And the value of the piece will be correspondingly less.
The term "centennial" is sometimes used to refer to pieces that are not antiques but are old and are done in a distinct antique style. For example, chair makers were making Chippendale style chairs in 1870. These were not the standard style of chair for 1870. The makers were consciously imitating the Chippendale style from 1770. If you purchased one of the 1870 chairs today, you would certainly have an old piece of furniture. And it would probably be well-constructed. But it wouldn't quite be an antique. Calling it a centennial is a way of referring to its age without overstating the case and calling it antique.
In the same way, even today, furniture is being constructed in older styles. A master craftsman today can put together some Shaker style furniture by hand. This can make for good furniture. But obviously, it doesn't make the furniture really Shaker or really antique. Instead, you would say that you have some "Shaker style" furniture. (And if it's still around in a hundred years, your grandchildren can call it "centennial".)
Lastly, remember that "collectibles" are not antiques, or not necessarily. An antique is defined by its age, its rarity, its uniqueness, and its conformity to the style of its period. Collectibles are defined by rarity and renown. A certain baseball card can be a collectible as easily as a certain figurine or vase. But unless the figurine or vase is done in a period style and is at least 100 years old, it doesn't meet the definition of antique either.