You can eat them at any age, but they get tougher as they age, and are best cooked in a crock pot or similar long, slow cooking. Around 16 to 20 weeks is probably the tenderest, but they are not full size then and (depending on breed) you may have just figured out for sure which are cocks and which pullets.
There is little if any difference between DP breeds as far as taste. Orpingtons are supposed to have the most dark meat, if that is your preference. I have read that Marans are particularly tasty. Personally, I've tasted several dual purpose breeds and cannot tell the difference.
Plenty of folks let them cross breed and have a yard full of "mutts," or "designer chickens"
About half of mine are mutts. Nothing wrong with this. If anything, it should be a good thing to have the genetic diversity, or "hybrid vigor."
One approach is to process the roos as they reach the 16 to 20 week stage, since a flock that's approximately half male, half female will be a real problem once they reach mating age. The girls will be way over-mated and suffer injuries, and the roos will fight and injure or even kill each other. It's often recommended here to keep about one roo to 10 hens, which I have found keeps the peace and generally keeps the girls in good shape. (Now mind you, I'm a relative noobie, just a few years.)
A breeder may keep the sexes separate and put one roo with only one or two hens when he wants fertile eggs, though. A hen will lay fertile eggs starting about 2 or 3 days after introducing a roo, and if you then take him away, she will lay fertile eggs for around 2 to 4 weeks. If there is always a roo in the flock, most or all of the eggs will be fertile -- though again this depends somewhat on breed. For example, some very fluffy butted breeds need feathers trimmed to mate successfully. If you stay with the old classics, though, they should manage just fine, like Rhode Island Red and Barred Rocks. My Australorps have done fine.
A fertile egg will not begin to develop unless it is incubated. This means it has to be kept somewhere around 95 to 100 degrees F, either under a broody hen or in an incubator. In much of the world, eggs for eating are not even refrigerated, just kept at room temp, and they do not develop. I eat fertile eggs every day and have never cracked one and found a half developed chick.
You might want to read in the meat birds section, especially on dual purpose breeds, A lot of that section is about cornixh X, which you can only buy as chicks. There are a bunch of threads there about breeding your own meat bird, though -- what breeds were used, etc. Actually, I would really recommend you do a fair amount of browsing around on here; an awful lot of wise and experienced people have contributed to this site.