Risk increases with age, and clinical signs are most frequently first seen in horses in their mid-teens.
However data collected from over 3100 horses tested for PPID during Boehringer Ingelheim's Talk about Laminitis campaign in spring 2012 have suggested that PPID could be common in laminitic horses as young as 10 years old - Veterinary Record 2012;171:239
Horses as young as 7 (Heinrichs et al 1990, Orth et al 1982) have been diagnosed with PPID.
Donaldson et al. 2004 gives the age range of horses suspected of having PPID as 3 to 28 years, with median age 15.5 (however some of these could have been falsely diagnosed in the autumn as seasonal reference ranges for ACTH were not used).
There may be differences between horses and ponies. At the 2011 BEVA Congress Andy Durham suggested that PPID may be a cause of IR in horses. Ponies develop PPID at a younger age than horses, suggesting that IR and hyperinsulinaemia may predispose ponies to PPID. In 744 cases of PPID studied, insulin was higher in younger PPID ponies than in older PPID ponies, suggesting that they were IR at the onset of PPID, but that weight loss which is frequently a symptom of PPID improved their insulin sensitivity.
Nicholas Frank suggests that PPID should be considered in any horse older than 15, but that ponies with EMS can develop PPID at a younger age (> 10) - ponies that become obese at a young age and then develop Equine Metabolic Syndrome have an increased risk of developing PPID when they reach their teens - EMS appears to predispose a pony to PPID.
It should be remembered that testing for PPID has improved significantly in recent years and that it has only recently been recognised that the majority (c. 90%) of laminitis cases have an endocrine cause.