A lot depends on how the car is driven. Over here, my two year old C-Max usually gets just short runs to the shops, so the battery tends to run down. This turned out to be the reason that the start-stop mechanism was not working. The battery came with the car, so it should be good for another year or two.
One thing to help with a cold start, that I saw somewhere, is to turn the headlights on for a few seconds and then start.