BB has also sent me the 1860 census for Iowa, which shows a Charles Brewster, born Ireland, merchant aged 44. This gives a DoB of 1816. He is living with his siblings, Eliza Jane, Mary and James.
BB has now sent me a death notice drom a newspaper dated 13th November 1893, which states: "Charles brewster, a resident of Fort Madison, Iowa, since 1844 is dead. He was a banker and 82 years old." This would give a DoB of 1811.
I prefer this as his DoB as he was much younger then: immigration record supplied by BB of a John Brewster arriving in Philly, aged 70, in 1826, along with a Charles Brewster, aged 13 = DoB of 1813.