Upvote:6
Every bill and coin ever issued by the US government in its history remains valid and will be valid while the government continues to exist.
At some point, the collector value of a bill or coin will exceed its face value, but with 2006 $100 bill, that will likely be decades or centuries from now.
Upvote:10
Yes, they're still valid, and should never expire. You may find that some places look suspiciously on the old designs for larger bills like that, but you can always trade them in at a bank at no cost.