Well, technically they are still lambs until they loose their first pair of teeth (the "milk" teeth) which can happen between 9 months to 13 months of age, it just depends on the individual. I think I would shoot for a weight more than an age. A lamb will dress right around 50%, sometimes a little less and sometimes a little more. So think about what you would like the size of the cuts to be and how many pounds of meat will make it worth your effort to process and just go from there. You could easily take ewe lambs and wethers up to a year in age to maximize carcass size, but I would harvest intact ram lambs before they are 6 months old. We have heard from commercial buyers that the packing houses really don't want to take ram lambs over 6 months old because they are more difficult to skin. I don't know if the skinning process is just a little slower than they would like or if it is sure enough tough to get done, but that is what we have heard. On the plus side, an intact ram lamb has the power of his natural hormones working in his favor and is going to grow and gain more quickly than a ewe or wether lamb. So it is rather likely that a good quality ram lamb will get to a respectable processing size before he is 6 months old. What the animal eats will have as much to do with flavor as anything and unless you are finishing your lambs out on a consistent feed ration in a drylot you will likely find that spring lambs taste a bit different than fall or winter lambs.
I just mentioned Dorpers because that is the breed of hair sheep that we have had the most positive experience with. They are very thick-made and are exclusively a meat breed bred to thrive in harsh, dry climates very similar to our's in the Texas panhandle. We have weaned higher percentage pasture-raised Dorper cross lambs that weigh 95 pounds at just 4 months of age, and we have purchased fullblood breeding rams that weigh nearly 200 lbs at just 1 year old. This is not the best breed to put on feed, but we have managed to get wethers on feed to gain just over a pound a day towards the end of their time on feed. The reason why hair lambs tend to have a more mild, less muttony flavor is because they do not grow wool and thus have less lanolin than their woolly counterparts. We have had several of our own lambs processed and the meat is similar to beef, but a little sweeter, much more finely textured, and in my opinion much better.
There is a whole list of hair sheep breeds that each have their own little niche, so with a little research I bet you can find something that will fit right in to your situation. Folks to the east in Oklahoma, Arkansas, and other places where it rains seem to prefer Katahdins. I don't know where you are located, but here is a little reading about a several different hair sheep breeds:
http://www.ansi.okstate.edu/breeds/sheep/hair.htm
For some reason the Royal White (aka "Dorp-Croix") isn't on the hair sheep list, so here it is:
http://www.ansi.okstate.edu/breeds/sheep/royalwhite/index.htm
Dorpers, Katahdins, and Royal Whites seem to be the most popular, but it seems that St. Croixs are gaining in popularity.