From the text:
At the beginning, when the laws of Animal Farm were first formulated, the retiring age had been fixed for horses and pigs at twelve, for cows at fourteen, for dogs at nine, for sheep at seven, and for hens and geese at five. Liberal old-age pensions had been agreed upon.
For a horse, it was said, the pension would be five pounds of corn a day and, in winter, fifteen pounds of hay, with a carrot or possibly an apple on public holidays. Boxer's twelfth birthday was due in the late summer of the following year.