Yes, I mean "Their lack of the knowledge"!
Look! a baby doesn't know the meaning of FIRE, it is because he doesn't have any fear of fire!
It's really funny, I know, but I personally think sometimes our problems are the result of our knowledge!!
we have an interesting poem in this case:
چه رنجی میکشد آنکس که انسان است و از احساس سرشار است
The tribulation is the result of sensibility!
Now if we accept this matter, a question would appear in our mind:
Is our "un-longevity" (tool-e-omr nadaashtan) and not having a young heart, the result of our much knowledge?
Or in other words: Does my grandfather have longevity and young heart because he doesn't know a lot of things that we know?
Or
what we know is not what we should know!!!?
I know, you're mixed up
I think it's a real philosophy discussion!
Anyway, forget about it, please explain the term "Young Heart".
What are the characteristics of a Young Heart?
In your opinion who has a Young Heart?