I came across an article awhile back written by a Dr. Jornigan entitled "How Old is My Horse in Human Years?"
The article was written 24 December 2013.
Dr. Jornigan says, "I get this question a lot. You know horses .. it's complicated. To estimate the age of your horse relative to a human, use the following formula: 1. First year = 12 human years; 2. Second year = 7 human years; 3. Next three years = 4 human years each; 4. Each additional year = 2.5 human years each.
(Source: S.E. Blackwell. The Senior Horse - More Than Just Basic Care in: NAVC Proceedings 2007, North American Veterinary Conference (Eds). N. Fort Myers, FL, USA.
Horse Age in Years: 1 = 12; 2 = 19; 3 = 23; 4 = 27; 5 = 31; 6 = 33.5; 7 = 36; 8 = 38.5; 9 = 41; 10 = 43.5; 11 = 46; 12 = 48.5; 13 = 51; 14 = 53.5; 15 = 56; 16 = 58.5; 17 = 61; 18 = 63.5; 19 = 66; 20 = 68.5; 21 = 71; 22 = 73.5; 23 = 76; 24 = 78.5; 25 = 81; 26 = 83.5; 27 = 86; 28 = 88.5; 29 = 91; 30 = 93.5; 31 = 96; 32 = 98.5; 33 = 101; 34 = 103.5; 35 = 106; 36 = 108.5; 37 = 111; 38 = 113.5; 39 = 116; and 40 = 118.5.
Borrowed this from another group I belong to....