[dghlsakjg]

I believe the U-6A Beaver is the oldest design still operated by the US military.

Initial design was 1947, still being used as tow planes for gliders in the Navy.

[dralley]

That feels a bit like cheating. B-52 is nearly as old and is legitimately widely used.

[abenga]

The B-52 is getting new engines and going for a full century of active service.