Why it's not correct? הגבינות הן ישנות
I thought it was difference between "the old cheeses" and "the cheeses are old".
It's not needed here, there's only one way to interpret this sentence since there's an extra ה in "the old cheeses" -
the cheeses are old = הגבינות ישנות
the old cheeses = הגבינות הישנות