There are so many variables. The answer will vary depending on many factors.
My daughter was 7 (almost 8) when she started riding lessons. She went to her first schooling show (with 4H) after 6 months of weekly lessons. Her horse was a school horse who had BTDT and took very good care of her.
Later she showed her Godmother's 23 year old horse. He was more spirited (even at his age) and gave her more trouble...but with just around a year of lessons, she was able to handle it.
Her horse (Acey) can be a brat. We have good and bad shows with her. Kitten is much more confident and handles Acey's moods well...but I'm so glad we didn't start showing with a difficult horse...I don't think she would have continued.
So, if you have a good horse, a good trainer, and self-confidence, you can show sooner rather than later. Add 3 months for every missing element.
Oh, and remember, showing is expensive! We average between $250 - $300 per show for show fees. (Trainer, gas, stall, entry fees, hotel, supplies). We only show smaller local shows. Kittens English show clothes average around $300 per year (she grows like a weed). So, don't show until you are financially ready too. :)
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