How old must the sire and dam be to registered a litter?

They are able to breed at 1 1/2 years of age, but most don't until they are 2 or 3.

The only way for a stallion to be a proven sire is to breed mares and see how his progeny turn out, phenotypically and even genotypically. A sire that sires good stock is one worth his keep; a sire that sires inferior foals is one that needs to be gelded. However, a lot of the time (which would be 95% of the time) an experienced horseman can tell if a colt will be a good stud stallion by studying his conformation and by looking at the colt's other siblings and half siblings from both his sire and his dam. A colt will show more of his conformation when he reaches maturity, when he has filled out his frame a bit more.