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Kimberly Archie’s son stopped playing football by age 15. But by then he had already absorbed many hits to the head during eight years of Pop Warner football and one year playing as a high school freshman.

By 2014, Ms. Archie’s son, Paul Bright Jr., was 24, and had done something out of character, his mother said: He bought a motorcycle off Craigslist, despite having no riding experience. He fixed it up, and rode it fast without a license or insurance. On Labor Day that year he T-boned another vehicle and died.

An examination of her son’s brain revealed previous damage to its frontal lobe, and he was found to have chronic traumatic encephalopathy, more commonly known as C.T.E.

Now, California state lawmakers are being asked to bar children under 12 from playing organized tackle football — a requirement supporters say will protect their brains from unnecessary injury and trauma. Ms. Archie, who had been working to make sports safer long before her son’s death, supports the bill, but thinks the age limit should be pushed even higher.