Five year old unopened boxed CPU – is the stock thermal compound still good?
It should be, given that in the same amount of time, it could've been attached to a CPU and has to enure that as well.
If you don't trust it, just scrape it off and apply fresh coolingpaste (which is different than those strips. The paste can actually dry out.
Journeyman Geek points out that if its slightly hard it should be ok, if it crumbles like dried toothpaste, you need a fresh coat of the stuff.