There is no exact science for converting horse years to human years, but one common rule of thumb is that one human year equals approximately three horse years. By that estimate, a 12-year-old horse would be roughly equivalent to a 36-year-old human.
This is only a general guideline. Individual horses age at different rates based on breed, diet, environment, and overall health. Additionally, horses and humans have different lifespans, so a 12-year-old horse may be considered middle-aged by equine standards but still relatively young compared to a human of the same age.