One thing I noticed when Lucas was picking Jake is how Lucas was smiling while he watched Jake's performance.
I have noticed that a lot of Jake' mannerisms as a young Anakin seem to almost mirror the way Lucas reacts to things in his inteviews.
Anyway, trying to make Anakin look like a younger version of Shaw wouldn't have made Shaw look more like a younger version of Shaw. If Lucas' point was to make Anakin appear as he did before he became Darth Vader, then no matter what they did Shaw would have been too old.