All designs of U.S. Federal Reserve notes remain legal tender regardless of when they were issued.
This means that notes from as far back as 1914 still remain legally valid for payments.
The US said this as part of its currency education programme;
Note that it is U.S. government policy that all designs of Federal Reserve notes remain legal tender, or legally valid for payments, regardless of when they were issued. This policy includes all denominations of Federal Reserve notes, from 1914 to present as per 31 U.S.C. § 5103.
The US government further went on to advise on how the old $100 bills can be authenticated.
Federal Reserve notes are redesigned primarily to make them easier to use but more difficult to counterfeit. This does not mean that older-design notes are not secure.
In fact, security features in older-design Federal Reserve notes, such as watermarks and colour-shifting ink, have proven to be so effective they have been retained and updated for use in newer-design notes.
To authenticate a $100 note issued between 1996 and 2013, move your finger along the note’s surface to feel the raised printing, tilt the note to see the 100 in the lower right corner change from green to black, and hold the note to light to see the watermark and security thread.
You should also see red and blue security fibers embedded throughout the paper, and microprinting within the numeral in the lower left corner and in the left lapel of Benjamin Franklin’s coat.
Magnification may be necessary to view the microprinting.