How old does something have to be for it to have an article?

I would say it depends on what it is since there's always a chance that it isn't actually lost and someone backed it up, like the social experiment Markiplier did or channels and groups dedicating to archiving media from people like Onision and VenomFangX.

The one exception to the rule that I can think of would be something that was caught via security cameras or on the news and whether or not they make it clear publicly that said footage won't be released to the public or was destroyed. If that's the case and there's thorough evidence for this, then I'd say to make a footnote for it and then play it by ear. I initially planned on doing this during the Penn State hazing trial when it was rumored that the event was filmed,but I opted out of it when I was hearing conflicting information and it felt like it would have been in poor taste with how things were still actively ongoing.

So, I'd say to weigh your options with what's in front of you and to get the vibe in the room first; especially if it's NSFW/NSFL or controversial in nature.