True, but in most fantasy settings, they are not stuck at the hunter/gatherer level, they are somewhere between antiquity and early modernity, and they stay there for many millennia, possibly beyond count (see also Martin's A Song of Ice and Fire world for another prominent example).

Colonization/terraforming does not work for me, I hate even the lightest mixing of technological space-faring with fantasy. Metals I don't want to renounce; though this raises another point for the scientists, is it likely that a younger planet would also have less metal, or less retrievable metal? Or at this time scale it likely does not make a difference?