When I was about 13 my dad bought some farmland that had a barn. The barn had the remains of about 20 bikes, all different sizes, colors, brands, etc but they were all old single speeds. My friend and I started creating bikes out of all the parts and riding them on the dirt roads and trails on our hill. Not a big deal when you broke something – just go back to the barn and replace the part or start over if you totaled it.
My first official mountain bike was a 2006 Trek 6500 SLR I bought on craigslist from a guy who had only used it on the street. I rode that until I destroyed the stock front fork and decided to get put my $ into a 29er instead of an expensive replacement for for the Trek. I gave the bike to my son, who uses it around town