Wing clipping--trimming the primary feathers on your chickens' wings--is not necessary unless your flock is flying into places they shouldn't be. Most breeds do not fly particularly well, so it is not usually necessary. (Juvenile chickens will typically fly better than adults, as they have reached nearly their full wingspan at that age--but not their full weight, yet.)
If you decide to clip your chickens' wing feathers for some reason, you will normally want to do that about once a year, after the molt. Some people recommend trimming the feathers of one wing only because this puts the bird off balance for flight. Young birds molt a few different times during their first year as they come into their adult plumage, so you won't want to clip feathers of very young birds. Plus, only feathers that have fully grown in should be clipped--DO NOT CLIP GROWING FEATHERS. The reason is that feathers that are currently in the growth cycle have a blood supply; cutting them can cause your bird to bleed, sometimes excessively. Clip only the primary feathers. To see which feathers these are, view this illustration .
Typically, we prefer to let our chickens have their full, natural feathers; however, in some cases, feather clipping does make sense---especially if flying over the fence means they would be flying into the territory of a neighbor's dog, flying into traffic, or getting themselves into some other dangerous situation.