The above is very good, but one important fact is missing. Junipers have two distinctly different types of
foliage. While your typical
bonsai owner is probably going to kill the plant before it reaches maturation since most people will be getting a spindly
little bush not a tree, that shouldn't stop you from knowing (
knowing is half the battle).
The juvenile foliage of a juniper is dense and has a scaled appearance, as if the scales were emerging from the bark like shingles. They are broader, greener, and more dense than than the mature foliage. I personally think they are more attractive, however it is impossible to keep them, because a branch that has been around for a year or more can turn into a mature branch.
The mature foliage looks more like a pine trees, the needles are distinct, they shoot more perpedicular to the bark, and they are thinner. It is on these branches that the juniper will create berries. This is very beautiful, because the berries are the same size as on a full juniper, and if you pretend you're one of those clay chinese wise men, the berries become big blue apples. That is, apples of the type commonly found on evergreens by clay chinese men who have had one too many poppy seeds.
The real challenge for a bonsai grower is not whether to choose mature or juvenile, because juvenile will become mature. The trick is to make the tree look good and real with both types. This came as a shock to me about three years ago when the first mature foliage sprung up on my three year old juniper (which had a difficult start in a tiny pot). I had my father clone several so I could try different things. While my first juniper has almost completely changed to mature (except the new growth, which I pinch back), I am now experiencing the same problem on my clones.
My solution to the first was just to let it happen. Until I talked to my grandfather, I had no clue of what was happening, and assumed the tree was dying because suddenly the foliage looked sparse and browner (or maybe less blue, as junipers typically are). He explained it to me, so now I am currently trying to figure out how to deal with this on my new trees. So far, I have removed the needles on a few branches to separate the two types, which looks cruel and is counterproductive. I have tried on one to continually cut it to ward off the mature branches (because after a good cutting or repotting, the tree resorts to junvenile branches) which has worked so far, but is probably not a good long term strategy. My best solution was to train the tree in a very tradition manner, with an upright trunk and horizontal branches, which makes the mature foliage look like a tree within a tree. However, junipers are very low and like to contort themselves, so this isn't really in the spirit of the plant, and it's sort of ugly because junipers look much better twisty.
Also, I have yet to force the tree to create needleless branches, which would probably be best, this way you can make groupings of juvenile foliage and as it turns into mature, you make it bare, so that you've got long branches capped in dense blue-green scales.