The simple, uncomplicated answer is that the expected lifespan of a horse is about 25 years. Ponies live longer.
However. In the UK we have a saying called "Donkeys' Years" - it means a long time. The average lifespan of a donkey is 45-50 years.
Mules live 30-35 years on average.
Also, the 25 year estimate is an average. I have known horses to live much longer, including one pony that made it to an estimated 48 (He had no papers) and was believed to be the oldest equine in the state of Maryland. I also know two horses that are both celebrating their 35th birthday this year. A well cared for animal with good conformation and genetic health can live longer, and advances in geriatric medicine are pushing the 25 year estimate closer to 30 - providing the horse isn't overworked at a young age, neglected, etc.
The horse year "ratio" is 4 to 1, but this method definitely breaks down. Here's a better pattern for a horse's lifespan:
Gestation: 11 months.
Weaning: 12 months in the wild, usually 6-8 months when domesticated.
Puberty: Somewhere between 18 months and 2 years. Unlike in humans, colts hit sexual maturity faster.
Full physical maturity: 4-6 years.
Old age: 18 years plus. (Some people put it at 15).
So, horses are fairly long-lived animals, with donkeys and mules living even longer.