Spoiler: Information regarding a story that can make that story less suspenseful or surprising.

Everything is a spoiler by this definition. For example, Keanu Reaves starring in John Wick is, strictly speaking, a spoiler. Someone who didn't know that could be surprised by Keanu appearing on screen (and possibly say "whoa" in response).

A person who says "that's not a spoiler" might be wrong in the strictest sense, BUT their most likely meaning is obvious: "that spoiler doesn't matter."

This presents us with a more subjective, but way more useful, definition:

Spoiler: Information regarding a story that can make that story less suspenseful or surprising in a significant way.

Is Keanu gracing us with his presence in John Wick a spoiler by this subjective definition? Everyone can decide that for themselves.

Spoil: Sharing previously unknown, or potentially unknown, spoilers with one or more people.

A person can spoil things for themselves (I sometimes spoil things for myself because the story in question is stressful in some way and I actually want to reduce the suspense).

Can you spoil something that's really old?

Objectively yes.

Many arguments that you can't spoil older things come with assumptions of similar age and experiences. Those arguments hinge on everyone being of a demographic close to or matching the person making the argument.

Even then, the actual arguments tend to be less "it's impossible to spoil it," and more "no one's obligated not to spoil it."

So it's possible to spoil something that's old, and the question is whether someone cares.

Sometimes, you shouldn't. Generally speaking, if someone my age hasn't seen Star Wars yet, the argument "if they actually cared, they would've seen it by now" seems fair to me. Star Wars has been widely popular, widely referenced, and came out before I was born. If I hadn't seen it by now, it would be selfish of me to demand everyone else stay quiet about (imagine the biggest of all Star Wars spoilers here) purely for my sake.

Not everyone is my age, however, and even if they are, exceptions could exist. It's really going to be contextual what's reasonable and what's just not caring about other people's feelings.