What is the opposite of spanish word viejo?

The opposite of "viejo" (old) in Spanish depends on the context:

For age:

* joven: young

* nuevo: new (can also refer to something being fresh or recent)

For other meanings:

* nuevo: new (can also refer to something being fresh or recent)

* moderno: modern

* actual: current

* reciente: recent

Example:

* Viejo coche: Old car

* Coche nuevo: New car

* Casa vieja: Old house

* Casa moderna: Modern house

The best opposite will depend on the specific situation and the intended meaning.