At 4AB Agility, we run Fun Agility Classes as opposed to competition agility classes (but if anyone makes it big, then a mention of your humble beginnings in your autobiography would be much appreciated).
We believe that if your dog is relatively healthy and not too reactive to other dogs, then he or she should be able to enjoy agility, whether you have a Chihuahua or a Great Dane.
Our classes are group classes but we take into account individual needs of the dog. For example, some dogs like a bit more room than others, so they will be afforded that option if neccessary.
Also, because we don't see ourselves as a competitive agility class, we are not governed by any strict jump heights, which means that we can cater for younger or older dogs that would struggle to make the "official" jumps. By keeping the jump heights down low, we can vastly reduce the impact caused by jumping repetition.