My week old relay node got this flag today, I couldn't find any explanations about it figured I'd ask here. All I found out was that it was introduced in tor 4.x
StaleDesc was developed throught a proposal. Anyone can send in a proposal to improve the Tor protocol. The specific proposal has the number 293 with the title "Other ways for relays to know when to publish". The implementation was tracked in ticket #26770.
Authorities recognize that a relay descriptor is too old and assign this flag. A relay should upload its descriptor when it sees this flag.
So when your relay sees this flag it should update its descriptor. This happens automatically. However if your relay is not able to update it, you should investigate why this happens.