"Distressing" isn't an emotional concern in this case -- it essentially means banging the banister up to make it look like it's been through a lot of physical wear over the years. Whack it with the head, claw and even side of a hammer to cause dents, taking care not to break the spindles loose. A bunch of heavy hardware such as nuts and bolts in a pillowcase or sturdy canvas bag, swung at the banister, also bang it up a bit. Chip away some of the detailing on the banister, such as small wooden rings, using a hammer and chisel, if you'd like it to look extremely worn. Sand over some of these areas, especially in places where hands might grab the railing, to make the banister look well used, rubbing off some of the paint. Add faux worm holes by using the smallest drill bit you can find, drilling tiny holes in clusters in random areas of the banister.
A banister, or any other object painted white, won't stay such a pristine shade over many decades, especially if outdoors. Distress the banister to beat it up a bit, then darken those areas by rubbing a dark-colored marker over them, then rubbing it in with a rag. Alternatively, mix a brown, gray or black latex paint with either water or glaze, creating a thin mixture. Brush or rub the glaze over the entire banister surface. The dark solution clings to the nooks and crannies, especially evident if you've done your work distressing the banister first.
Varnish of one form or another has been used for generations to seal and protect both painted and unpainted wood. Over time, sunlight and just plain old age turn that once-clear varnish a shade similar to honey, with tones of yellow or orange. Recreate that look by mixing a honey-like color latex paint with water or glaze -- the more glaze or water, the more translucent the final finish. Brush it on with a foam brush, then rub off excess with a rag.
Create the look of many years and many coats of paint on that white banister by adding wax and more paint. Rub paraffin or an old white wax candle all over the banister; this helps additional paint layers come off later. Paint the banister a color slightly different than the existing shade, or vastly different, as if the home and the banister went through stylistic changes over the years. Rub another coat of wax over the banister, followed by yet another shade of paint. Repeat this as many times as you'd like, then sand through some, and sometimes all, of the layers in various places on the banister. The end result looks like an aged banister with hunks of paint missing from its various painted layers.
Sometimes, old paint finishes crack and chip, creating a look similar to crazing on old china. Create this look using a method similar to the wax and layered paints technique, substituting crackle medium for the wax. Paint brushed over the crackle medium immediately cracks, showing the layers beneath it. This effect is especially striking if using contrasting colors, such as a dark or bright color over white. Crackle medium is available at craft stores and specialty paint shops.