Not knowing how far you are along in the book, I would encourage you that there will be an instance in which one animal gets very close to retirement and becomes injured. This is interesting because of the lack of ability to serve the final working months and watching how the leadership will deal with such a circumstance. It is not clarified how injury at a late stage is dealt with ever. So the animals then watch the circumstance that transpires and realize once again that things are really never what they thought they were going to be.
You can find the answer to this right at the beginning of Chapter 9. So if you need more details, look there.
The different animals had different ages when they would be allowed to retire. This depended on how long that particular kind of animal lived. So the geese, for example, could retire when they were 5. But the horses and pigs had to wait until they reached 12.
There were rumors about a part of a field being set aside for retired animals. And there were rumors about how much food they were going to get. But no one had actually reached retirement age yet so it was not clear as to what would actually happen.
To answer this question, take a look at Chapter Nine of Animal Farm. In this chapter, it is stated that the issue of retirement had first been settled when the laws of the farm were developed. Specifically, this involved setting a fixed age at which each animal on the farm can retire:
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.
It is also agreed that each animal shall retire with a small pension, and that retired animals shall have their own grazing area, formed from a section of a larger field for pasture.
These plans for retirement, however, never come to fruition. No animal on the farm ever retires. In Chapter Nine, it is hoped that Boxer might be the first animal to enjoy his retirement but, instead, Napoleon sells him to a glue manufacturer.
Retirement, then, is a dream which Napoleon quickly crushes. He has no intention of ever letting the animals rest: he is far more interested in profiting from their labor and selling them once they have become too old or sick to work.