Though that said, most law enforcement has a statute of limitations on crimes except the very worst of them like killing another human being. If you're going to go after a fossilized tweet, just remove it, locking accounts after that long seems rather silly unless they are posting new content against the rules.
Even with law enforcement, there is something called statute of limitations for most minor crimes.
Funnily enough, in law enforcement there is similar concept, in US it's called statute of limitations.
Depends on the juristiction. The law of England and Wales, for example, does not have any time limitation for prosecution.
Twitter isn't law enforcement though, and it would be a tough sell to hold them up to the same standard.
According to the screenshot, the author is free to have his account unlocked after the tweet is removed.