When an adjective is used to represent a group of people, such as the sick, the poor, the blind, the young, the unemployed, the elite, or the damned, the noun phrase conveys an almost absolute quality and a general meaning; it is used to refer to all the sick people, all the poor, all the young, etc.
The comparative noun form, the richer, works if the subject being compared, in this case the rich, has been mentioned previously or is clearly understood from context.
The rich cry but the richer cry even more.
There is a very well-known aphorism which says
The rich get richer and the poor get poorer
but here richer is an adjective; therefore the definite article is not required.
Furthermore, without context, I'm not sure how the following sentence would fare
The younger face even greater challenges in the world of work.
It's understandable, but the comparative noun phrase version doesn't sound idiomatic, it's not something that we hear, the reader would have to fill in the missing context. A better alternative would be
The younger the unemployed, the greater the challenges they face
Here the comparative form “the younger” is used to head the noun phrase “the unemployed”, it modifies the generic meaning of all the unemployed people