That's par for the course for a Down breed - start off with a single (gives Mama the opportunity to raise a lamb without having her first-time milk production over-stretched), then go on to twins and end up with singles again in old age (when twins would see her struggling to keep in good condition). Just be careful not to feed the single-bearers too much - lambing a large single is much more trouble than tangled twins. Our flock, with an even spread of ages, generally scans at around 165%. I like to have as many old ewes as possible, provided they're in good condition (and it's often hard to keep condition off the Southdowns), as they know exactly what they're doing and just get on with it.