Complex sentence: J.F. Harrison, the man later largely responsible for the design of No.71000 "Duke of Gloucester" was the popular choice, but at 42 years old, it was considered inappropriate for someone so young to take the post.
Split into simpler sentences: J.F. Harrison, the man later largely responsible for the design of No.71000 "Duke of Gloucester", was the popular choice. However, at 42 years old, it was considered inappropriate for someone so young to take the post.