I recently had a lot of the same questions. I'm 22 years old, and was also wondering "how old is too old to start boxing." Well, that all depends on your goals. Like Von said, if you have any aspirations of being a world champ, at your age, and mine, we are probably too late. Unless we find we have some god given talent (i'm still digging around tryin to find mine lol, i KNOW its there! )

For amature fighting however, give it a go man. You'll likely spend a bit of time (year or more give or take depending on your fitness level) training, getting in shape, and of course learning how to box before you might be ready. As I am only a month into boxing, the following information is only what I hear from some reputable sources on this website.

Amature boxing, in the U.S. is broken down into age and skill catagories. 18+ would be where you and I fit in. Then, they further seperate the field between those with less than 10 fights, and those with more than 10 fights (the "Open" division).

Again, that is what I have gathered in my short experience in the boxing world.

As to how many people you would see in your weight class, I can only be slightly less helpful with that question. I would imagine this would have to do with where you live and where you train. Here, in southern NJ, the lighter guys like me (I'm 5'10 130) seem to be the minority. I'm not sure if this is true of all areas, but it is in my gym at least. When sparring, weight class matters signifcantly less than when boxing competitivly. And assuming you do get to a competitive level, by that time you shouldnt have trouble finding fights in your weight class. Again, take this with a grain of salt. Someone more knowlegeable here please correct me if I'm wrong (or lemme know if I'm right )

Now, for your last question. This one I can answer with the most certainty. I know for a fact many gyms make you wait a fair amount of time before you spar. I've heard up to a few months before they decide someone is ready. This again, will vary according to A) Your natural ability and fitness level, and B) The gym and the trainer. At my gym, I was sparring after my 3rd one hour class. And have continued to spar regularly since. Now, not everyone at my gym spars yet, although a lot of people have the desire to. There are those who arent ready even after a month or two.

Hopefully that answers all of your questions. If you have more, feel free to ask. Also, someone correct me if I'm wrong with any of that.