My twin sons are 21 years old and have been cutting their own hair for a few years. I had an old hair clippers set that didn't work very well so I bought a new one and it works a lot better. It has different parts that you attach for the length you want to cut off. Travis cuts his hair very short so it's easy. Dustin leaves his hair a little bit longer but does a great job. To do the back of his head he said he looks in a hand mirror and stands with his his back to the bathroom mirror. He's very particular about his hair and has to have it just right. He will trim some hair and come and ask me if I think the sides are even or if he needs more cut off in the back. Some people have thought he gets his hair cut professionally. You would never know either one of them cut their own hair.
When my boys were little they had beautiful curly hair. Dustin's was blond and Travis had brown. I don't think they got a haircut till they were three. Then the curls disappeared as they grew older (I still have some of their curls in an envelope I saved). I used to cut their hair when the boys were little up till they were in grade school. Then I started taking them to a barber till they were in high school. Cutting your own hair saves a lot of money and the boys cut off just how much they want. With my hair style I couldn't cut my own hair; but at least the boys do!