As conformation is an evaluation of breeding stock, I see an altered class as pretty pointless, although I can understand why there's an interest in it (showing dogs IS fun!)
Veteran classes at specialties are always a bit of fun however, and I like watching the older dogs do what they love doing.
Thats what I plan on doing. Showing my future dogs for a few years then having them fixed. What is the average years for a show dog? When is a good time to retire them?
It totally depends on the breed, and how fast that breeder's particular lines mature. Most Shibas finish their championships very early on in their lives, usually before they reach 2 years of age. People often don't special a Shiba much longer than its 5th or 6th birthday. In other breeds, the dogs aren't even all that competitive until they're 4-5 years old (I know this to be the case with some lines of Shetland Sheepdogs).
As far as showing a dog and then altering it, as Gallien Jacks pointed out, most breeders aren't going to be selling you a fabulous show specimen knowing that you're going to finish the dog's championship and promptly alter it. They're much better off keeping the dog themself, or selling to someone who's really serious about showing and breeding the dog.
If you aren't keen on breeding a dog, a good idea to look into is perhaps getting a dog that is a retired champion. Many breeders rehome some of their retired show dogs, so they don't end up spending their best years sitting in a kennel. Getting one of these guys would allow you to compete in a few Veterans classes, and get an idea as to whether or not you're really into the whole "dog show game," without having to own an intact animal.
Also, keep in mind that a good breeder is a mentor- if you are truly interested in showing, they won't leave you hanging, they will nurture and guide you along with your new puppy. No breeder will expect a complete newbie to finish a dog, and then breed a litter, care for a pregnant b!tch, raise and sell puppies. Most of the breeders I know maintain the breeding rights (at LEAST) on their b!tches, so that they have an active say in where their lines are going. In my case, my breeder co-owns my b!tch, and when the time came for her to have a litter, she did *everything* (being a college student, I really wouldn't have time to effectively care for puppies). That being said, I got to babysit the pups for a couple weekends, and ended up taking the show prospect puppy home several times to work on socialization. That puppy is now an American champion (currently 1/2 way to his Canadian Ch.), and the pride of my heart (he's the boy in my siggy)- it's really quite a special thing.
With all this in mind- not every dog that finishes *needs* to be or *should* be bred. There are many dogs that finish their championships that aren't bred, or just don't end up cutting the mustard- and this is good- I certainly don't advocate breeding *every* animal that finishes. It's just that if you're buying a puppy, no breeder will immediately dead-end the dog's career without knowing how the animal is going to turn out.