Complex sentence: Hugh Willoughby's father died 12 June 1715 age 34 at which point whilst an infant and below the age of majority Hugh Willoughby became 15th Baron Willoughby of Parham.
Split into simpler sentences: Hugh Willoughby's father died 12 June 1715 aged 34. The infant Hugh Willoughby became 15th Baron Willoughby of Parham.