I believe that I last painted the door in 2004, the year we moved in. When we bought the house, the garage door hardly closed and looked as if it would fall off its hinges. We must have replaced it promptly, for that fall we were storing things in the garage. That is how I figure it, anyway.
If the paint store mixes a can for you, the can will have a sticker giving the name of the color, the mix of pigments, and the date mixed. I had a look at the old paint can, but since black is a color always stocked, there is never a need to mix it, and so no sticker. There was a can number on the top, but the cashier at the store said that he couldn't use it to establish an age.
The can specifies a 25-year limited warranty, if the paint is properly applied. I am sure that a good painter could find faults in my application in 2004 and again in 2015. I don't know that the 2004 faults contributed to the flaking of the paint. I think that the door has been hard on the paint. For one thing, a garage door has joints. Ours has four rows of eight panels, and a good storm must send water through between the rows; the panels have water stains on the inside. For another, it is pulled up a track and lowered down, traveling six feet each way. It is a shaky trip, and was shakier for a while when the wheels were failing and we didn't know it.
It seems to me that the true craftsman must know how much paint to put on his brush or roller, and get it right every time. I might, on a good day, get this right four times out of five. The true craftsman must prepare his surface correctly, and here at least I am better than some people we have hired, who would paint over loose dirt. On the other hand, the professional will know how to set priorities, and will spend most of his time on what is most visible; the amateur can spend great effort on corners, and then lack the energy to prepare the main walls.
A neighbor, a contractor who learned his trades in Europe, says that Americans don't want to pay for good painting. The fliers put out by Fine Paints of Europe, a Dutch brand, hint at the same by describing the apprenticeship that Dutch painters must go through. I expect that my neighbor and the Dutch are correct, but I must say that telling who will paint well is harder than they might think. (Well, my neighbor would say "Hire me", and he would be correct.) The contractor who says all the right things Friday might show up Monday with a couple of guys he just hired in the Home Depot parking lot. In general, we paint what we can reach with no more than a 10-foot stepladder.