The interesting thing about my saguaro is that it is not in Arizona. It's at my parent's house in south Texas. Back when I was still in grade school I got one of those little greenhouse cactus seed kits while on vacation at Carlsbad Caverns in New Mexico. I'm not sure exactly when that trip was but I am guessing it was between 10 and 15 years ago.
After a few days of searching, I was almost convinced that it wasn't a saguaro because many sources said that a ten year old cacti would only be 1.5 to 6 inches tall. As you can see from the photo above, mine is between 2.5 to 3 feet tall! But after doing some more research I found that those heights were for wild cacti in the desert. Since Texas is slightly wetter than Arizona (and since my parents probably watered it occasionally) my saguaro probably grew faster than it would have in the wild.
So all these things together have led me to the decision that my cactus is a saguaro! After living here in Arizona where saguaros are a big deal (a full grown one can cost thousands of dollars), it's fun to know that I grew one without really trying! And that's even more special since it's in south Texas, which is not really it's native habitat (there are no wild saguaros in Texas or even in New Mexico). It's kind of cool to think that my parents are probably the only people in my hometown with a saguaro in their yard!
(By the way, if anyone who has some cacti knowledge thinks that this might not be a saguaro, please let me know why not and what you think it might actually be. I'm no expert so it's very possible that I'm wrong!)
Update 9/29/11: I've posted an update with photos of how the saguaro has grown over the past two years.