I thought a small tree could be topped and transplanted. I had no idea they would be that old...
This is the way nearly all bonsai are made. Let the tree grow until the trunk is the diameter you want, chop it at an angle and then let it back bud. Let the buds grow and train one into the new trunk. Sometimes they're chopped again. The scars from the chop heal over very slowly. This is really the only way to get taper in the trunk. Otherwise, you'll just have a straight pole.
Evergreens are actually done the same way, except they're chopped just above a low branch. The low branch is then trained upright into the new trunk. You can do this with deciduous trees as well, but it restricts where you can chop the tree.
And whoever asked about Juniper - many of the trees sold in box stores are made from juniper. They make good bonsai. But many, many types of trees are can can be used. The best trees to use have naturally smaller leaves, short internodes (spaces between branches), and fine twigs. Maples, beeches, boxwoods, oaks, apples, pines, junipers, cypress, elms, and ficus are all commonly used.