Age 
The age of a guard llama does matter. If a guard animal is needed immediately, then a younger llama is not appropriate. Llamas do not begin guarding until about 2 years of age or older. A younger llama will not be territorial or mature enough to properly defend a flock of sheep. Buying a younger llama as a guard animal and placing it in the flock is acceptable if there is not a current predator problem. A younger llama may be less expensive than an older experienced guard animal.

Sex 
Both male and female llamas will naturally guard against predators. No difference has been found between the guarding ability of female and male llamas. Geldings are used most often because there are more male llamas who are not put back into breeding programs. The male llamas are castrated because they may try to breed ewes in the flock and injure them. Purchasing a guard llama of either sex would be fine. 

Wool 
Fiber coverage is a factor for a guard llamas. If you are interested in having some quality fiber to use, you may want a heavy wool llama with good quality fiber. Harvesting that fiber will require some maintenance like grooming throughout the year and yearly or every other year shearing. A light wool llama is a better choice if you do not want to be bothered with it. The climate you live in is also an issue. If it is very cold, a llama with a little more fiber would be a better choice and conversely less fiber would be a better choice in a warmer climate. 

Testing/Experience 
There are a few programs which actually put llamas through a series of tests to evaluate their guarding ability and then sell llamas guaranteed to guard. These will be the more expensive guard animals, but it may be the best option for a flock with a current predator problem. You may also be able to buy a llama who has already been a guard animal. This may be much harder to find and price may depend on the age of the llama. Most who get guard llamas decide to keep them. 

Prevention or Current Predator Problem 
If you are just trying to prevent having a problem with predators, a llama of any age will be sufficient. The vast majority of young llamas will develop guarding ability between 2 and 3 years of age. There is a small percentage who will not develop guarding ability and it will take time to find a replacement. If you do have a current predator problem it will be necessary to bring in an experienced guard animal. 
 

Any guard llama will still need to be able for you to handle as to perform routine tasks like vaccination, deworming and nail trimming.