I bought my daughter a weanling when she was 10. My daughter started riding at 9 months, before she could walk. She got her first horse at age 3 and rode all the time. There was never a time that my daughter worked with her filly by herself. I was always out there, guiding her when she would do training sessions. The hardest part, as
@Knave said, was waiting. She hated waiting. At age 2, her filly looked strong and grown, but I made her wait until her filly was 3 to ride her. My daughter did a very good job of training her filly, who soon will be 10. We have now had the mare for 10 years. It can be done. But not by someone who doesn't know what they are doing. Then it will be super hard.
It still can be done. My friend was 9 when she convinced her parents to buy her a 9 month old colt. None of them knew how to train a colt, and it was very rocky and hard. They made a lot of mistakes. But that colt turned into a very talented jumper. He was a registered quarter horse. My friend showed him at FITS in Florida and all around Texas doing open jumpers. I hope you are as successful as my friend and my daughter. I wish you the best.
Here is a picture of my daughter and her filly.
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And now grown.
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