The older they get especially peaches, the less likely they will throw new scaffold branches. If you planned to train as an open center, I would not wait or use existing branches. If using existing branches you can wait. I would never cut off side branches as they so tend not to grow new ones, peaches are the worst at not throwing new branches. You can let branches compete for the scaffolds, no need to cut any off early on. If using other systems of pruning like modified leader, then pruning is different. I’m not really familiar with any other method except open center, which is all I use.,. I don’t grow apples.
24 seems low to me, 36. If heavy squirrel pressure, higher branching is in order.
Not sure, but some distance is desirable. Although I have trees with none that are fine.
A photo would be helpful. I had 3 ft tall Red Haven trees and I cut them back to about 16-18 inches. There were 4 branches below the cut all facing different directions. Open center is easy if you start early. too late and it requires big cuts. Look at Olpeas’ trees, very low and very open
For me, my goal is to never need to use a ladder to prune or pick. So why waste height on trunk. Critters will get to yours trees regardless of height. Deer will eat your trees to the trunk if not protected by fence/cages.