Up until this year, we didn't bother to tag our sheep until they went for slaughter or to the ram. Many shepherds don't. But this year we are tagging them at about 4 - 6 weeks old. Partly because you are supposed to and partly so that we can keep a check of which lamb came from which ewe so that we know who has done well, and can eliminate genetic problems etc. I certainly wouldn't tag them any younger then that, whatever the regulations are, because it is painful and I couldn't do it to a tiny lamb.