How old is old? What breed are they? Those two things together will influence the answer to your question. 'Old' varies from breed to breed and type to type. I don't keep commercial sheep but they don't seem to be used beyond about 7, whereas the Primitives, as long as they have their teeth, can go on lambing until well into their teens.
A shearling has no experience so you have no idea what kind of breeder she will be, but an older ewe, depending on why she is being sold on, should have a few years of lambing under her belt.
So I would advise to ask for far more information on the particular ewes on offer, so you can make a decision based on knowledge rather than on general guesswork.