Depends on the horse, the rider, the coach and the trainer.
I know people who have been riding for the same length of time as me (sometimes longer) who I would not even let lead a green horse while I feel quite comfortable doing first rides on most horses.
With the horses I have one who after 6 mos u/s was able to have a green rider. Others it might be not until they are 10, 15, or even 20+ that they can handle a green rider.
Sometimes the trainer or coach can go so slow with the horse or rider, that it takes 10 years. Sometimes, it's 6 mos.
So an extreme combination of factors.
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