It is best to use eggs no more than 10 days old. After that, the "hatchability" goes down.
Chicken eggs take 21 days to hatch, from beginning of INCUBATION at 97 to 100 degrees Farenheit. So, if you gathered eggs daily for six days, and then put them into an incubator, your 21 days starts the day you put them into a heated and stable incubator. They need to be turned at least 3 times a day until day 18, after which you LEAVE THEM ALONE until they begin to hatch.
Humidity varies, but the general rule is 40-50% humidity for the first 18 days, then bump it up to 65% or more.
You shouldn't try to hatch pullet eggs, the first eggs laid by young hens. Wait a couple, three months at least, until the eggs are larger (because they start out pretty small). And, of course, you need a rooster to fertilize the eggs!
Hope this helps!