I am afraid to say that neither is correct - and it is not clear what you are trying to say in either sentence.
In comparison to their ages’ littleness, they learn how to live big.
In comparison to their ages littleness, they learn how to live big.
I am afraid to say that neither is correct - and it is not clear what you are trying to say in either sentence.
I'm not sure what you are trying to say but I guess it's something like this.
In spite of their youth, they have learnt to behave like adults.
Frank