The Jedi Order took Force-sensitive children at a very young age (typically until the age of three, but preferably earlier) to start training them to become Jedi. A notable exception to this was Anakin Skywalker, who was accepted for Jedi training when he was just under 10 years old, which would ultimately prove to be the downfall of the Jedi. A midi-chlorian count through special testing could determine how strong one was in the Force. There were also other testing technologies developed to hunt Force-sensitives during the Great Jedi Purge.