You can!

To find out the age of an object, right-click it, Edit > More > More > Inspect.

Watch as it digs up the creation date — it'll examine all prims in a linkset, making it useful to see when there's multiple creators involved. Thanx to Cube Linden for this feature!

(Yeah, it's buried 3 layers deep, and is part of a larger "first-use metadata" project, so like so many other things Second Life, it's subject to change.)

Let me know if you find anything older than this, it's "The Man" statue @ Philip's Hill in Natoma.