No, it is too risky to let a child whose mind has not fully developed make a decision that you cannot come back from. They should be able to do things like cut their hair and/or let their hair grow out, but not go too far with these things.

You aren't "fully developed" until around the age of 25 on average, so saying you are fully developed the day you turn 18 but no earlier is just wrong. People aren't just making these choices one day like "oh I'm going to be trans!" like people do when cutting their hair. They go through doctors and psychiatrists for screening with these things. It obviously isn't a good idea to jump the gun and let a child do whatever they want, but with a medical expert's point of view even a twelve year old could confidently get treatment for any major issue, trans issues included.