Viejo vs. anciano vs. antiguo
The Spanish words viejo, anciano, and antiguo all basically mean old, but are used in different contexts. Viejo generally refers to old age in persons or things, anciano specifically refers to an old person, while antiguo refers to something old in time or ancient.
Viejo
A1Viejo can refer to an old person, usually someone around middle age or beyond. It can also refer to objects, meaning they are worn out or not new.
Mi padre es muy viejo.
(My father is very old.)
Ese coche es viejo.
(That car is old.)
Anciano
B1Anciano is more respectful and specifically refers to a very old person, similar to the English term elderly.
Mi abuelo es un anciano.
(My grandfather is an elderly man.)
Los ancianos necesitan cuidados especiales.
(The elderly need special care.)
Antiguo
A2Antiguo usually refers to something old in time, like an object, building or custom from long ago. Something is antiguo if it has been in existence for a long time.
Este edificio es muy antiguo.
(This building is very ancient.)
Es una costumbre antigua.
(It's an old custom.)
Summary
Though all three words – viejo, anciano, and antiguo – have the basic meaning of old, they are used differently. While viejo can refer to both old persons and objects, anciano is a more respectful term that specifically refers to very old people. On the other hand, antiguo is used when referring to things that are old in terms of time or history.