You can verify the age by checking a canine tooth. Remove a tooth and its surrounding jaw bone with a pair of side cutters. Boil in a pot for 10 minutes then pull the tooth from the alveolar socket. Check the tip of the root. If the pulp canal of the tooth is still open the animal is less than 9 months old. If the root tip is closed then it’s an adult.
Got this female coyote in a dirt hole set the other day. Beautiful fully prime animal. Weighed 25.1lbs. Extracted canine shows that the pulp cavity is still open at the root tip, so that indicates this year's pup, probably born in early to mid May. Frank's 25 pound rule is pretty much dead on from my experience (pups tend to be less than 25 pounds by fall/early winter) if the animals are "western-type" morphs. I also have the second morph in my area. What they're calling eastern coyote. These animals probably have more wolf genes in them and they are a bigger animal. Dentition is a little more robust, wider skull with slightly higher saggital crest on the skull, and I find the feet tend to be a tad larger. The pelts are slightly longer. Instead of 40-4 lb males for the western type, I've trapped the second kind with weights usually five to eight pounds heavier. Big males are 48-52lbs, based on about 300 animals I've weighed over the years. Based on what I've trapped, by examining the root closure on the extracted canine, I've had pups that were up to 32lbs during the November - January trapping season. Every once in a while you'll get a heavier animal when it's obvious that the coyote has regular access to dead stock. I have sheep farms on both sides of my farm and I see some crazy fat coyotes some years. Lots of bone powder and sheep wool in the scat and the farmer is not losing animals to predation.
Here are some coyotes I just pulled from my drying boards. There are two adults and two "young-of-the-year" coyote pups. You can see that November/December pup pelts are not much smaller than the adults.