OK, this came up in a pub quiz we did last year. And knowing what hogget was won us the cash. So just in case you find yourself in a similar position...
Lamb is the meat from the ovine ('sheep' to you and me) that is less than one year old. It has a mild flavour and low in internal and external fat. Requires short cooking times.
Hogget is the meat from the ovine that is one to two years old. It has a more intense flavour than lamb, low to medium in internal and external fat. Requires longer cooking times than lamb and more suited to roasting, stewing and braising.
Mutton is the meat from the ovine that is more than two years old. It has a very strong flavour, is high in internal and external fat. Requires extended cooking times and more suited to curing, 2nd class roasting (slow and low heat), stewing and braising.
So, now you know my little lambs.