I think its worth noting that currency forgery is a business that operates on scale. Forging one $100 note is an enormous amount of work for not much payoff. The goal is to forge 1000s+ of $100 notes.

But at that point, you run in the question "Where do you spend it?" Unless your forgeries are exceptional, you can't just drop them off at the bank, precisely because banks will become suspicious if you show up with a duffle bag of 50 year old notes.

Fortunately, we have a number of methods for laundering currency - either by spending it where people don't look to hard or by mixing it with more modern notes so the 1000 notes you just printed don't stand out in a pile of 100,000s of such notes. Strip clubs, casinos (this was the original plot of Rush Hour), liquor stores, and other cash-heavy businesses do a great job of co-mingling fake and real notes. Black market trafficking is also a popular venue for disposing of dirty cash, as drug runners and smugglers are notoriously unsophisticated in detecting forgeries.

So, old bills can and do work as counterfeits in the right venues. And eventually, you'll see them vacuumed up and disposed of at the Federal Reserve right alongside the legit older currency. It's just a lot more work to do correctly.