Conversation so far:
What was Guy Carleton, 1st Baron Dorchester's early career?
In 1742, at the age of seventeen, Carleton was commissioned as an ensign into the 25th Regiment of Foot, in which in 1745 he was promoted lieutenant.
Are there any other interesting aspects about this article on Guy Carleton, 1st Baron Dorchester besides his early caree in the 25th Regiment of Foot?
He became a friend of James Wolfe; he may have served with Wolfe at the Battle of Culloden during the Jacobite rising of 1745.

Question: What was he promoted from?
Self-contained rewrite: What was Guy Carleton, 1st Baron Dorchester promoted from when joining the 1st Foot Guards?