Hi!
A bony end is normal of old piggies; hip bones and spine become more prominent as piggies age.
You measure whether they are still a good weight for their size around the ribcage ('feeling for the BMI'). While the very elderly generally lose muscle tone and gradually lose weight, please have your piggy vet checked if you have any concerns - the weight loss may be due to an underlying issue that can be stabilised or treated. Weighing once weekly is never more important than in old age. When the body is no longer able to process and absorb nutrients all that well, an elderly piggy can be very hungry in an attempt to make up in amount what their failing body is no longer able to achieve in quality; that is why judging weight just by eye can be deceptive.
Weight - Monitoring and Management (Includes a chapter on how weight changes over a life time, what exactly to look out for when feeling for the BMI around the ribs and when to see a vet re. weight loss or sudden weight gain, which can indicate a fluid build-up from congestive heart failure in the elderly).
Impaction - How To Help Your Guinea Pig.
Guide contains videos and practical care tips, including dietary ones that can help to ease impaction in the earlier stages. Please supplement an impacted boar with vitamin B as they can no longer process their caecotrophs (the poos that contain the digestible but not yet fully broken down fibre for the second run through the gut) but a very regular, hay based diet can help ease the build-up.