@Rob.t - Bonsai is normally an outdoor hobby. Tridents are fully hardy outdoors in Las Vegas in winter, and with some shade protection can be grown outdoors in summer. The reason it is an "outdoor hobby" is that most young seedlings and cuttings going through the phases of development can be 5 or 10 times the size of "finished size" of the bonsai. You're question is "when to start training?"
Now.
How large a diameter trunk would you like? Thumb size?, wrist diameter? Leg diameter?
Generally little or no pruning is done until the diameter of the trunk is near the diameter you envision for the finished tree. This means the tree needs to grow, TALL, often tridents are allowed to get 10 to 20 feet tall in order to get the proper diameter trunk.
Then they are cut down to roughly one third of the finished height. You want a tree 12 inches tall with a 4 inch diameter trunk, you allow the seedlling to grow until the trunk is roughly 4 inches in diameter, most tridents will be between 12 and 20 feet tall at this diameter. Then you cut the seedling down to roughly 4 inches. The seedling will then explode with back buds, pick one as the next segment of main trunk, allow that to run until it is 2/3rds the diameter of the trunk below it, cut it down to 3 inches in length, it will explode with back buds. Pick the next segment of trunk. Allow this to run until it is 2/3rds the diameter of the previous segment, cut to 1 inch in length. The whole tree will be full of back buds, now pick some of these to be branches, and one to be the new leader.
You can see how this process needs space and height to grow out. More than the average light set up will allow. This is the reason bonsai is usually described as an outdoor hobby. You can increase diameter of trunks with horizontal running branches, it is the total surface area of leaves that a trunk supports that determines the caliper of the trunk that develops. You need a large trunk caliper to create the illusion of age.
Your seedlings are ''ready'' to begin training in that they need a period of rapid growth, they need to pick up size, in order to develop trunk diameter (caliper).