There are two totally separate issues here...

There is a criminal law regarding falsifying an FAA form. This is a federal offense, with possible fines and prison time associated. Most crimes (except murder) have statutes of limitation. Not sure what it would be, but I would guess 5-7 years for something like that? Best to research it and find out for sure.

As far as FAA regs, those are admin law and I doubt there is any statute on that. I know that once the FAA starts an investigation, they must complete it one way or the other in a timely manner. This is not law, but rather NTSB legal precedent...they effectively require a "speedy trial". But I suspect there are no limitations which would protect your medical and pilot certs from revocation if a serious past omission came to light.

Better see a lawyer if your question is not academic in nature.