I think it depends on the breed and how they do on your pasture.
A good doer, like a Mule, will probably get too fat over its second summer, so would be best sent off at 14-16 months, once it has put back on the condition it lost over winter but before it gets as fat as butter on summer grass.
A slower-growing animal, like a primitive, probably needs the whole of the second summer to fully mature, so would be 18-19 months. Or could be kept on for a third summer and sent off at 28 months-ish.
We kept a couple of Swaledale wethers on the hill (moorland) for two years, sending them off as the grass reached its best in their 3rd summer. They were awesome.