Hello Scremmit,
there are much more experienced and wiser voices than mine out here but.... seems to me that there's no right or wrong approach on this question, and many variables....
so, some lambs are sold for slaughter at 4 + months old - generally fast growing, larger breeds....
the 'rarer' or more 'primitive' breeds are often much smaller sheep, and take much longer to grow, generally just on grass, to get to a weight for slaughter (we have Balwens, and we don't take our 'lambs' off until they've had 'two summers', and so are about 18 months old....), so some lambs are taken off at 10 or 12 months, or even at 18 (technically called 'hogget')...
also, lambing takes place at different times....so, you could buy 'last year's lambs', and they may have been born last April, or in December.....
maybe sensible for you to work backwards.... sort of... decide if you want to take 'lambs' off to slaughter at the end of this year, and then decide what breed you might like (or what's available and/or flourishing in your area) and then work out what age you should buy them in to fit in with your plans...
good luck!