Complex sentence: At the age of twelve Lord Bristol appeared at the bar of the House of Commons and pleaded for his father, then in the Tower of London, when his youth, graceful person and well-delivered speech made a great impression.
Split into simpler sentences: At the age of twelve he appeared at the bar of the House of Commons and pleaded for his father who was then imprisoned in the Tower of London. His youth, graceful person and well-delivered speech then made a great impression.