The aged look of an old oil painting can be emulated to really make your work get an extra ring of authenticity.
Because a finished oil painting consists of several layers, (the canvas, 2 or three layers of base coat gesso, any number of layers of oil paint and then several coats of varnish) the whole thing can be a bit chemically unstable. Each layer may well dry at a different rate. Because of this, expansion and contraction of the painting will result in a crazing of the varnished surface. This pattern of small cracks is known as craquelure.
In addition the surface layer of varnish will darken with time. The simplest way to replicate this darkening is to wash tea over a finished canvas. A simple rub with a wet tealeaf will instantly add a patina of age.
You can also add a craqulure over the top of the varnish. A pot of crackle effect varnish can be purchased from most DIY stores.
Old frames also age, so don't forget to age your frame for a more cohesive finish.