He is a new stuff. << It's plain simple and easily understood.
He is an old stuff? << Depending on the context, could it have any sense that he is old in his age, or only that he has been working there for a long time?
I think you meant "staff" because I've never heard "stuff" used for calling people. It would be funny and inappropriate, actually.
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I think you meant "staff" because I've never heard "stuff" used for calling people. It would be funny and inappropriate, actually.
kenny1999He is a new stuff.
Something is wrong here. It's not as simple and easily understood as you may think.
kenny1999He is an old stuff.
This is equally strange; however, there is the expression "old stiff".
Someone on stackexchange has posted something on the subject: