is hard working an adjective?
Yes, if you use a hyphen as in these sentences:
He is a hard-working young man.
That young man is very hard-working.
https://www.onelook.com/?w=hard+working&ls=a
He is a hard-working young man.
That young man is very hard-working.
https://www.onelook.com/?w=hard+working&ls=a