That is an excellent question to which there is no easy answer. Non Vintage (NV) doesn't have a vintage date and very few of them show when it was disgorged* on the back label.
If you buy from a store with a slow turnover of stock it may already be quite old, but from a shop that sells well it could have been released in the past month.
Technically, Champagne has to be aged before release and the Champagne houses will tell you its ready for drinking then, but I am convinced extra time in bottle noticeably improves the wine.
*final stage in making