I've had my ducks for just over a year...Not Mallards but Rouen & Pekins. They were born last June and started laying this spring around the end of February or March. My Rouen has already gone broody and is in the process of hatching out (we've got 4 so far).
Sorry can't give you specifics on mallards but hope this helps...
Don't give up hope! I have a four-and-a-half month old Cayuga who just started laying, and she has laid 5 eggs in 5 days! I think it just depends on the individual duck.
Mine started at about 8 months, been very steady since then. At about a year, this past spring, 2 of them went broody! I don't have a drake though, so it was pointless.