Complex sentence: Robert Cantu, a neurologist, believes that children under 14 should not play tackle football, since their brains are not fully developed, and myelin (nerve cell insulation) is at greater risk in shear when the brain is young.
Split into simpler sentences: Robert Cantu, a neurologist, believes that children under 14 should not play tackle football. Their brains are not fully developed, and myelin (nerve cell insulation) is at greater risk in shear when the brain is young.