"You are not old!"
Actually, nejseš comes from Central Bohemian dialects (now Common Czech) and only later came to Moravia.
We teach standard Czech here, not any dialects. In the very last skill of the tree we show some Common Czech.
The above (nejsi stará) is feminine. Masculine is nejsi stará, plural (masc. animate) nejste staří. See the tips and notes about the hard adjectival declination.
You mean for neuter? Like Ty nejsi staré (dítě)? It is super weird to address anyone in the neuter gender. But even that would require NEJSI or NEJSTE for the second person.
Otherwise I can't see any other reason you might have in mind.