Complex sentence: When he became president of the LDS Church, he was the oldest person to attain that office up to that point, which continued until Gordon B. Hinckley reached Smith's equivalent age in June 2006 (Hinckley continued as president for another 19 months).
Split into simpler sentences: When he became president of the LDS Church, he was the oldest person to attain that office. He was the oldest person to hold that office until Gordon B. Hinckley reached Smith's equivalent age in June 2006 (Hinckley continued as president for another 19 months).