Before the early 17th century, chairs were comparative rarities, their comforts enjoyed by only the privileged few, while everyone else made do with stools or benches. A hundred years later, the situation had changed significantly, and many of the kinds of chair popular today -- the armchair, the dining chair, the easy chair -- had started to become available. Over the centuries, the appearance of chairs altered in tune with fashion. However, the style of a chair isn't the easiest or most reliable method of telling its age. This is because by the middle of the 19th century, cabinetmakers (the term for furniture manufacturers) were already producing reproductions of chairs in earlier styles. For beginners, the best way of identifying an old chair is by looking for general signs of age and considering how the piece is manufactured.
Inspect the piece for a credible build up of dirt in its nooks and crannies, the kind that would happen over a long period of time. Even a chair that has been very well cared for will have spots that were missed during spring cleanings. A chair that is covered with loose dirt in an indiscriminate way has probably been artificially aged.
Examine the chair for signs of wear. As with the presence of dirt, this should be credible -- i.e., reflecting the kind of life you might reasonably expect the chair to have led. Look at the front legs for little dents and nicks where they have been caught by the heels of past sitters. Also turn the chair upside down and scrutinise the feet. Many antique chairs would have rested on cold, damp unheated floors, so look for signs of discolouration and even rotting as well as scrapes and dents. If a supposedly antique chair has very fresh, clean-looking feet, this means that it's either a modern reproduction or that the rotten parts have been cut away, a serious flaw in its own right.
Consider how the chair has been manufactured. Peer into any loose joints. If you see dowels -- cylindrical pegs used to join two pieces of wood together -- then the chair dates to no later than the early 1800s. Squeeze any padded upholstery. If you feel the presence of springs, then the chair can be no earlier than around 1850. Look to see if the chair sits on casters -- little wheels to make it manoeuvrable. Combined with other indicators, these can be useful rough indicators of age, with brass casters pointing to a 20th century date, porcelain casters to the 19th century and casters made of loops of leather to the 18th century.