The only tainted sheep meat I have tasted was from a 7yo tup, and that was pretty vile. It was very oily, with a strong taste, hard to disguise even with loads of curry spices.
We routinely send our entire Hebridean males off at 16 months, from summer grass, and they are as sweet as can be. I have wondered about Shetlands though as they can smell quite 'rammy' as they get older, so perhaps that might continue on into the meat. Has anyone else noticed if a smelly, rammy lamb goes on to be smelly, 'rammy' meat?