Maybe I am out of my league here, but in Phantom Menace we learn how dangerous it is to train older individuals to be Jedi. Therefore, since that happened before the original trilogy with Luke, where we also learn training too late in the Jedi ways is considered dangerous, the groundwork seems to be pretty well laid that the longer you wait to train the force, the better chance it is used for evil instead of good.
It has more to do with the fact that when you're younger, the Jedis can mold you into the type of person they want you to be. Anakin was already the person he was that could not really change, and if he's power hungry and envious, then its almost impossible to eradicate from him. So yeah, older people would be more prone to the dark side since they aren't molded from birth.
I think Darth Maul's origin is that Palpatine got him as a young child and raised him, and thus was completely loyal to him. Palpatine molded him into exactly what he wanted him to be. Vader and Dooku came later in life, and thus while they could be corrupted, they were not loyal servants to him and plotted on overthrowing him from the start. So unless you want an apprentice that seeks out to kill you, for both Jedi and Sith, the younger the better.