Same here - I've had a 15 yo produce big healthy twins and rear them with no probs. On the other hand I've had much younger ewes who start to lose their teeth at 8 and that is usually the deciding point for me. A ewe will survive fine with no incisors, but not rear twins too.
Part of the criteria I use for a good line to breed from is longevity, by which I mean a ewe which can still rear lambs, has all her teeth, good udder, good feet, good condition, etc well into her teens.
Your 11 yo ewe may be perfectly fit to go on for a few years more, or she might be past it already. Have a good look at her, especially her teeth.