This is not what he said. I know a lower skill will train faster than other related skills that are already higher. But he said that it is more difficult to train a skill from 7-12 than from 1-6 in general. Thats what I understood.
That is what I thought he meant as well, and as far as I know it is wrong. The only reason it would take long is because the skill would train faster if it is lower than other related skills, or because the player is going to be older when you are training from 7-12 in general.