Two second google versus an actual shepherd.
A slightly longer google tells me that lamb grown in the UK or in New Zealand is typically pasture-fed and slaughtered at around six months. I did find a
survey being conducted right now for Defra as it appears that the actual data isn't currently available here in the UK. The survey assumes that six to seven months is typical, as stated here by our actual shepherd, but wants to produce more accurate figures.
There is a form of lamb called milk-fed lamb, which is lambs slaughtered much earlier and only fed on milk. This is something that is eaten in Spain and Italy. It isn't really done here - and you're not going to buy it by accident as it is more expensive. You would have to buy it from a specialist butcher as it has to be imported from Spain.
ETA:
That survey is actually a decade old. I searched for the results but couldn't find them. However, weight at slaughter is recorded, so that assumption of six to seven months as typical is not based on nothing - it's based on weight at slaughter, which is going to be a pretty good guide. Again, as our resident ex-shepherd said, funnily enough.