How to say 'old' in Spanish; there's 2 ways: if you're referring to the masculine, 'viejo'. Try it. If you're referring to the feminine, you would say 'vieja'. Try it.
If you want to say something like 'my car is very old', you would say, "Mi carro es muy viejo." Try it. In the feminine, if you want to say like 'my bicycle is very old', you would say, "Mi bicicleta es muy vieja." Try it.
Those are the 2 different ways: 'Old' in the masculine or 'old' in the feminine; viejo or vieja. That's how to say 'old' in Spanish.