When we first joined the Hebridean Sheep Society, way back when, there was a photo in the Year Book of a 13 yo ewe with triplets and the wonderful double entendre caption of: "Broken down by Age and Sex"

Love it. We've never had triplets from Hebs, though they do pop up occasionally.
Jezzie had all twins during the time we had her (from 2001) apart from one single. She rarely had tup lambs, just the famous Gladstone Laughing Boy. She kept all her teeth until her last pregnancy, when they started to go mid-winter. We didn't breed her after that, as although she had excellent conformation still (with the help of extra feeding), we didn't feel the extra demands of twin lambs was worth her life. Eventually she just didn't wake up one Christmas Day.
I think how long you go on breeding your ewes depends on various factors. For example, are you breeding for meat or breeding stock? We produce breeding stock so longevity of both tups and ewes is important in a breed which is reknowned for long lives. If you are growing for meat production then you need efficient lambers which either raise twins every year on good ground, or thrifty singles which can be reared on marginal land unsuitable for other uses.
If you are just breeding from a few pets, to be lawn mowers and an interest for the children, with some meat for the freezer, then your interest is more in the animals themselves, so you can keep on breeding from them for as long as you like, and let them carry on into a sedate retirement. Economics is less of a concern.
Also there seems to be a big difference in the natural longevity of various breeds, especially how long they keep their teeth.