in short: The size of tree you want to create defines the trunk: 6 to 12 times the trunk thickness = height of your tree.
The age is irrelevant. The older the trunk the better te bark. But in a general store you will only get plants that are a few years old, so do not worry about it.
Main things to look for are, in order of relevance:
- Leafs that are naturally small or reduce well (So the right species)
- nicely spreaj surface roots
- A good tapering trunk without clear bumoos, swellings etc
- Small branches on the trunk (For conifers particularly)
- A trunkline that pleases you
Everything else is just a matter of styling afterwards.
Repot in the right time of year, and when you know the plant is healthy. Untill that time, adjust your care to the substrate available. (That being said, I repotted 6 japanese maples today because the substrate was bad, and I could not wait any longer; I wanted the plants to partake in the automated watering setup.