Complex sentence: At age 12, Yonattan was diagnosed with retinal dystrophy, a chronic degenerative condition; by age 14, Yonattan's vision had been reduced to less than two percent.
Split into simpler sentences: At age 12, Yonattan was diagnosed with retinal dystrophy, a chronic degenerative condition. By age 14, Yonattan's vision had been reduced to less than two percent.