Also, what are you giving for mineral? A deficiency wouldn't cause them to be thin, but it can lead to them being unthrifty.
Absolutely vet a fecal sample to the vet to check for worms and coccidia. Have they ever been wormed? Ever been giving a coccidia prevenative? Yes, they're older than the age in which a coccidia preventative is given, but if they never got one and have a high load, it can cause severe problems, specifically with nutrient absorption.
If they're still smooth and sleek, a fecal clears them of worms and coccidia, then they just aren't getting enough calories. Increase their alfalfa, add grain or beet pulp, something to help them to increase their body condition.