Very common.. US Ticonderoga class cruisers were first commissioned in 1983, so they're all ~40 years old. We're in the process of decommissioning some of them now, but many will likely still be sailing in 2040. Often times older ships get retrofit with the latest defense/offense capabilities and new propulsion if there's something they'd benefit from.
Ohio and Los Angeles class submarines were mostly built in the 1980s as well and many / most (?) are still active.
Otherwise, the oldest active ship in normal duty appears to be the amphibious command ship USS Blue Ridge (1970), the oldest carrier the USS Nimitz (1975), and the oldest submarine the USS Ohio (1981).
Ohio and Los Angeles class submarines were mostly built in the 1980s as well and many / most (?) are still active.
https://en.wikipedia.org/wiki/Ticonderoga-class_cruiser
https://en.wikipedia.org/wiki/Ohio-class_submarine