While it is a good idea to begin jack russell training when the pup is still young, it does not imply that grown dogs are hard to train. Grown dogs, just like puppies, are capable of learning new lessons taught to them so long as they're given the chance to do so and that correct training strategies are utilized. In fact, grown dogs are not too difficult to handle because they're less distracted therefore they are able to pay attention to the training. If you consider taking home a grown dog in the near future, here are a few places you may want to take a look at:

Shelters
Animal shelters are where most pet owners turn over their pets when situation demands it. Unlike the popular belief, not all dogs under the care of a shelter have problems. They're turned over commonly due to their owners. It could be that the owner is moving and it's impossible to keep pets in the new place, separation or divorce, almost no time for training or exercise and several other personal reasons.

Breed rescue
Like shelters, breed rescues take in abandoned dogs. But they do not just take in each dog in need of their aid. They scout animal shelters, take in the breed of their choice and put them up for adoption after assessment. So if you specifically want a jack russell terrier, jack russell breed rescue is your best choice.

Breeders
Breeders do not only place puppies, a few have grown dogs too who would do good with appropriate owners. A jack russell breeder will happily let you take the dog home so long as you are able to provide the needs, like high quality dog food and correct jack russell training, in order to keep him happy and healthy.

While these places or people need responsible and loving new owners for the pets, you cannot simply take home the dog instantly. Know that they will be asking you questions and perhaps visit you at home to determine whether or not you'd make a responsible dog owner. Bear in mind that jack russell terriers are high energy, fearless and vocal dogs so you must be willing to deal with these characteristics.