Yep, it's just a parent thing. My mom, who is decidedly non-horsey, still hates it every time I go tearing off across a pasture after cattle...and I'm about to turn 29

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Like others have said, be grateful that they do worry about you because that means that they care and try to obey their wishes. You'll be old enough soon enough and be able to make your own decisions on how you ride (even though they still won't like it LOL).