There are two types that I know of so far, regarding OLD SOUL.
The first is pretty obvious with people 'aging' before their years due to substance abuse. They look tired and old from all the wear and tear they put themselves through. They look into the mirror and wonder where the years went by?
There are those that hold onto something from the past and it too can 'wear' a person down and age them. Revenge or bitterness because of something that happened to them when a child. It goes 'round and round' down through the years and weighs heavily upon their soul, like a 'Ring' that needs to be "let go" (Frodo). Sometimes things like this are basis for cultures and their existence.
Then there is the person who is just into old stuff or things of the past be it for physical or mental reasons. Ancient civilisations, Prehistoric eras, old furniture, etc. I find this a more positive aspect of OLD SOUL. Kids who grow up with more of an influence by old people, than young, tend to exhibit that 'old soul' influence. OLD SOULS can also be known as 'Late Bloomers', people who come into their own in the latter part of their lives.
Oh well, I guess that's really 'three' then.
Because Humanity is still developing as a species. Our minds are still developing too and I'm pretty sure there is room for improvement in ways that no University knows or has recorded. I love Neurosurgery stuff and how the brain works. Being in the Theatres holding people's heads while the Surgeons screwed in large braces with sharp points into the skull was pretty neat too.

Coming off 'Anti-Depressants' in a cold-turkey way, I was able to experience absolute 'hallucination'. I've seen patients experience such with various other medicinals, like Tramadol. Watching them reach out for something and their eyes seeing something that isn't really there.
The Mind can play some amazing tricks.
People who have done damage to the brains via long usage of alcohol and narcotics also 'see things' and hallucinate, but I believe this is due to a lack of the brain to 'relate' to the reality around it ...well, that's a lengthy subject.
My hallucinations were, to me - as real as could be. Only my common sense and knowing the circumstance that I was going through kept me from ...well, I soon came to my senses. I would close my eyes, but in my mind - I was still awake for all sorts of nightmarish experiences to unfold.
I did this on a number of occasions (in relation to my counselling as well ) - as a sort of 'experiment' upon my self.
Of hallucinations, tricks of the mind - happening because of no substance influence and correlate to the circumstance at hand. Well, sometimes 'past experiences' in relation to dangerous situations come back as that '6th Sense' which is now commonly recognised by many people who work in dangerous professions, like Rescue Workers. Its like the peripherial vision of the mind - allowing you to see/recognise/remember something from the corner of your eye at a glance. Body Memory is also a major factor that contributes, where your hand darts out like reflex and does something before you even thought about it or had time to think about it.
I think people use more than 10% of their brains. Different people use different parts, like an Artist would use the part most for 'visual', while a Politician would use the part more to do with 'speech'. Einstein was a smart man, but stupid in many other ways. He just used the part of his mind that he needed the most. I sometimes wonder 'how' smart he was, considering he used to work in a Patents Office where he had every opportunity to go through everyone's 'Theories & Ideas' as a basis for his own stuff - of which he was just about 'Theorism' himself

Its debatable.
I reckon Stephen Hawkins is a good example of a Brain & Mind that is exceedingly developed. I say this, because I reckon it compensates him for his lack of 'physical' ability.
Something lost is something gained.
Lots of Deaf people do Art because what they lose in hearing, they enhance in their sight.
Like Blind people can hear a pin drop and a lot become Musicians.
This is what I call "Following your strength, not chase your weakness".
To me - physically handicapped people who enter Para-Olympics, I tend to see as those who chase something down the track that they lost long ago.
Maybe if Oscar Pretorius went for some University Degrees rather than Gold Medals, he may not have been stupid enough to shoot his girlfriend on the toilet?

But its all about 'choice' and if people want to, via technology or whatever, get back what they have lost - well good for them.
If someone told you that you can't swim or you will die.
Would you fight it to prove them wrong or say "Ok, I'm off to get my Pilot's license instead." ?
Both ways are worthy.