Thousands of Boston’s young adults who only have a high school education now have a place to meet with advisers who can help them pursue financial aid for college or training for a career.Mayor Martin J. Walsh yesterday formally launched the Connection Center, an office at the Ruggles MBTA station that aims to offer free education and job counseling to the 4,747 Boston high school graduates age 20 to 24 who have no secondary degree or path to a career.