1. As soon as you can plug it in is when you should be using a fan. In my opinion anyways...

2. No, it wont stunt thier growth or stress them unless you add wind too late...

- Technically it's the stress the moving air puts on the plants that benefits them. This will make them stronger, bushier, and much more vibrant. But this has to be done before they get too big.

3. I HIGHLY recommend putting a fan on your plants ASAP.

When grown from seeds I would recommend turning the fan on and leaving it on even as the germinated seed hasn't even sprouted through the soil yet. This allos the moving air to stimulate them as early as possible.

Technically, the earlier the better.

The reason for this is because you don't want to stress the plant out. You do, but not in a traditional sense. When done earlier in the plants life, obviously it's not a drastic change for the plant to adapt to, which could cause a negative stress on the plant.

The earlier you start with your natural variables (Wind, Light, Water, Natural soils) in your grow-space the better off your plant will be because your plant will be naturally adapted to its environment. Anything else added to the environment afterwards needs to be added gradually and gently so you do not shock the plant. Sometimes shocking a plant wont phase it, other times it can kill it. This is basic stuff...

But basically, yes, earlier is better, especially with something as small as wind...