Did I miss the passage of a new law? This law would of interest to the Presidents that have been elected to Congress after serving their terms as POTUS, such as Adams and Johnson.
Well, if 60+ years is "new", then yes. I believe that it came with the same amendment after Roosevelt died that limits the POTUS to two terms.
That would make it a few years after Adams. If the Johnson you mention is Lyndon, then he never served in Congress after being POTUS. He refused to run for what would, legitimately, have been his second term and died about 4 - 5 years after leaving office. If you mean Andrew Johnson, then same answer as for Adams.