The high schools around here have MTB teams. That seems like the appropriate age to get serious for most sports. There was a Grom race series for younger kids this April-May, though with only three races it didn't look like it was a big consistent program.
Folsom Grom MTB Series
Folsom Cycling | Home of the Bulldogs
https://www.facebook.com/broncosmtb/
Norcal High School Cycling League
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Genesis 49:16-17