Children have better neuroplasticity, a fundamental requirement when learning something so mentally nuanced as wielding the force. Also, being so young they don’t form unhealthy attachments, they’re separated from family and “raised” by the Jedi order. Don’t forget what happens with unhealthy attachments…. Anakin.
They still had the belief of childhood. When we are young, we believe all sorts of things that, as we age, we stop believing in. If you take an older child to train, you have to “untrain” him from all the garbage maturity has taught him…what is and isn’t possible. And since people all have different experiences, it is impossible to have one way of training that will untrain older people correctly.
@smudges If there is a 1 in 999 trillion chance that somebody “might” reference sex in their comment, @RedDeerGuy1 is going to mark the Q as NSFW!!! ;]