It also depends on the breed.
In a lowland situation most mules are done by 4 because there teeth have fallen out or something, lleyns can be routineley kept until 6 and so on. I have ewes who raised lambs just fine for me and are 9 odd. I wouldnt cull on age alone, I tend to treat each ewe on an individual basis, I gob and bag all of those over 4 pre-tupping and I kick out any brokers, lumpy udders or poor doers and go from there. I like to know who has raised lambs for a long time because I would like to keep her daughters.
Re: buying full mouthed ewes, I think it is a splendid idea, unless you are looking to put them on a hill. You can buy on farm from a person who has a similar system to you and you know they have raised lambs for x number of years hopefully without problem. Try to buy from someone who is getting rid because he is keen to bring some of his home-bred young replacements into the flock (I know this is hard to ascertain the truth of 100%, but going to his farm to look about should help). Buy them at a tenner over cull price, get a couple of lamb crops out of them for replacements etc and flog them as culls and your flock depreciation will be minimal.