The older the bill the more scrutiny it will receive so unless it's perfect someone will figure it out.

Old bills are eventually removed from circulation over time to the point where counterfeiting older forms of currency is ineffective because it is no longer accepted as a form of currency.

That's why old currency isn't valid after it's been changed though? That's why most countries have swapped to plastic notes at the same time as adding anti forgery things.

I remember paper money. In my country there was a period of transition from paper to plastic money where both were valid forms. I had a similar thought. I made some old paper money on a normal printer, but I found an excellent material to use for the woven shiny ribbon. I gave the counterfeit notes to my friends at school for a laugh and they used it to buy things from the dairy and tuck shop (school cafeteria).

Actually worked surprisingly well, until the glue used to join the two sides failed. One note pulled apart and the tuck shop owner noticed while cashing up. The police came around to my house but we placated them with muffins. It turns out someone had tried flushing a bunch of notes down the school toilet and the police tried to retrieve the money from inside the plumbing system.

Fun times. I was about 11 years old. Had no idea of the seriousness of what I was doing. Cops found it pretty funny, but I remember they were still very concerned about the notes spreading into circulation.