One perspective is that it if archaeology is the study of humanity "through the recovery and analysis of material culture" (wikipedia), then everything where that is done qualifies, and could be quite young. E.g. WW2 is clearly in scope: while we do have documentation and eye-witness reports for a lot of it, they are far from universal and on-site studies of battlefields, former prisoner camps, sites of war crimes, ... are done and contributing to what we know. And historians in general are dealing with even younger events, e.g. historians are studying the cold war (although I'm not aware of any archeological studies of it - maybe there are some of the actual wars during it?)

On the other hand, just because an investigation of a somewhat recent event is digging through material doesn't make it archaeology, but I guess that's partially distinguished based on motivations? (e.g. accident/crime investigations etc)

EDIT: younger example: https://www.archaeology.org/news/8291-191213-south-carolina-... "Archaeologists are investigating a possible Vietnam War–era military training site at Fort Jackson, an initial entry training center for the U.S. Army in South Carolina, according to an ABC News report." - I guess discarded beer cans could be useful dating something like this

It varies by jurisdiction, what the research goals are (if any), and the details of what's found. 50 years plus for human remains is a pretty common number in my experience, but objects can really go all the way up to the present.