A U.S. soldier serving in Vietnam in 1970 kills his commanding officer with a fragmentation grenade (a common strategy dubbed "fragging"). Would the soldier be non-liable after a certain period of time, or could he be charged with murder even if he was identified as the killer, say, seventy years later?
Under 10 USC 843(a), i.e. Article 43 of the UCMJ,
A person charged with absence without leave or missing movement in time of war, with murder, rape or sexual assault, or rape or sexual assault of a child, maiming of a child, kidnapping of a child, or with any other offense punishable by death, may be tried and punished at any time without limitation.
The lack of limitation in the case of murder has been in Art. 43 since the UCMJ was enacted in 1950.
There is no statute of limitations on murder
This is true of other serious crimes as well like sexual assault, war crimes, genocide etc.
The person can be charged now, 10 years from now, 100 years from now. Assuming they are still alive, of course, you can’t charge dead people.
As a practical matter, historical crimes are more difficult to prosecute due to loss of evidence, both physical and testimony (because witnesses are dead). For that reason (among others) a prosecutor may decide not to proceed.