"My father is not very old."
Translation:Mi padre no es muy viejo.
Yeah that's really tough, and just takes time. I like to think of it as "state" vs. "attribute," and that seems more clear to me. So for example, being old isn't a state (like being tired or sad or in a certain place would be), but is an attribute of the person. So if someone's deaf (es sordo) or bald (es calvo), because you're talking about general attributes. But if someone's in a state of sadness, "está triste" ... or if they are currently here, "está aquí." Hope this helps....
I agree with you, but his age is not a long lasting attribute.... Not like being ugly, or tall or such... Consider the idea of being dead - in Spanish it is esta muerte - but I have never heard of death being a temporary attribute or short term condition. I know ultimately you are right, but there are times when it is a tough call!
"Tan" doesn't really mean "very." Tan can either mean "as" in a comparative construction (e.g. "tan viejo como tú," "as old as you") or it can mean "so" in the sense of "so amazing" or "so old." So just as in English there's a subtle difference between "really old" and "so old," Spanish does the same.