I remember it was somewhat available in the late 1970s and early 1980s where I grew up in New England. Its availability was sporadic but my mom bought some if she saw it. I've eaten it. Not my favorite thing. There are other countries where it is not marketed, either, for example where I live now, in Spain. Lamb that is younger than what we can buy in the USA is a tradition. At Christmas it's popular to roast a whole suckling lamb and there are restaurants that specialize in serving it. It's about the only sheep-related meat you'll find here. Not a gram of mutton to be found anywhere. There are plenty of sheep, of course. I think here when sheep die or are slaughtered they wind up being turned into pet food or something.