I think this is the bigger thing, and I recall a reviewer I enjoyed online commenting in a similar vein-George wanted Anakin to be all these things, a pilot, an inventor, a dreamer, a love interest, and it was too much to cram in to this character starting at 9 years old. The dynamic between Anakin and Padme is off from the beginning because he is trying to infuse so much weight in this relationship.I think George was so dialed into making Anakin as young as possible while still being able to competently interact with adult characters that him being 9 and the equivalent of a child mechanical genius was something on which he was unwilling to budge. The younger and more innocent he is the more tragic his eventual downfall. That's definitely a point which can be argued since as I posted above Anakin could have been 13, still a kid and still traumatized by being separated from his mother whom he's forced to leave behind in slavery. His road to the Dark Side would still be the same and his childhood wouldn't need to be jettisoned, only advanced a few years to the onset of puberty.
But George being George his way ruled the day. I guess we're lucky that Jake Lloyd was so endearing and convincing as a child Anakin that everything else can be reasoned out and accepted if reluctantly, because had George cast the wrong child actor we'd be complaining about far more than the age difference between Anakin Skywalker and Padmé Amidala.