I'm leading my D&D party through an undiscovered land, where, as far as they know, no one comes back from. I want them to encounter an isolated tribe of as few people as possible -around 30 maximum. I figured they'd be what's left of much larger group of people after around 800 years. The lore is that a magical cover had been set around the terrain to hide something inside. Whoever crosses it, soon dies. The area covered is vast (~1000km radius) and so a few cities and villages have been under the spell as well.
I'd want them to be the citizens of one of them after 800 years of being cut off. Is this viable?
Their bodies would, I assume show some deformations , they would be weak and prone to illnesses, right?
They'd be illiterate because of lack of 'the elderly' in recent generations (they'd die pretty young due to the illnesses and genetic illnesses). Also I see them as childish and utterly immature for the same reason: their parents would die too young to show them how an adult behaves and well, not knowing it either since they (the parents) would die before 25 when the brain matures.
Since I added some assumptions I repeat the question: is it viable? Where am I mistaken?