Daisy is 18 and Gatsby is 27 at that time.
"Her white roadster was beside the curb, and she was sitting in it with a lieutenant I had never seen before….His name was Jay Gatsby."
"The largest of the lawns belonged to Daisy Fay's house. She was just eighteen."(chapter 4)
and since Gatsby was 17 in 1907, he is 27 in 1917.