Well, somebody somewhere always starts the pruning. I would like to know when that happens.
I would say everyone, everywhere start pruning their trees. At least within the bonsai hobby.
The key word is trees. Your question was about seedlings, they should not be pruned. They should be left to grow into mature trees.
So when to start pruning a tree that have been growing from seed? That depends on species, growing conditions and the final result one have in vision. There are no set rules that works in every occation. But your seedling will need more than 5 years in any case. That is plenty of time to read up on the species, learn how to grow it healthy, watch styling videos, talk to people that grows the species locally and to make a vision for the design of the tree.
But for now, just focus on growing a healthy tree. If you are eager to start pruning, get yourself a mature tree and start pruning. That will get you into the bonsai hobby 5-10 years faster.