21again.com
40again.com many years back.
Is there commercial value in the name? Yes, for anti-aging stuff for instance. Therefore an end user - health blog / affiliate marketer may find it interesting BUT there is no keyword in it.
Go to Sedo, list it. Put a fixed price on it maybe in the range of $500. Enrol it in their partner program (that allows your domain to be sold through several outlets).
To resellers, I am not sure whether they will like it. Opinions about names containing/starting with numbers vary. Won't be worth much to them.