I let both of my boys start when they were 12. They climbed in the backyard and with a harness on at all times. I only allowed them to climb when I was present. That summer they wore the bark off the tree up to 12-14 ft. Once they understood the mechanics of safe climbing, I then started to introduce common problems that occur. Key thing was to have a quality harness and get hooked up as soon as they stepped into the climber. They developed trust in the harness when I would bump the foot part of the climber away.
That fall I climbed a tree next to the one son as he used the climber for the first of many trips.
Safety first and foremost.