My problem with this is that it excludes the people that don't play to level 40. I for one start a new character long before that and would be majorly disappointed if I never got a chance to kill a full grown dragon so I would say level 10 for dragon killing. It wouldn't be easy but it is doable.
You seem to be under the impression that level 40 will be exceedingly high, as it was in Oblivion. However, in one interview, one of the creators described the leveling process to be more like a progression from level 1 to level 50, rather than from level 1 to level 25 as it seemed in Oblivion.