Awesome program, but it also depends on the unit you join. Been in it since the age of 11, when I joined the Navy League Cadets (for kids under 13), then moved up to the Sea Cadets when I hit 13 1/2. Had tons of leadership roles, attended Petty Officer Leadership Academy (POLA), participated as a staff cadet during summer trainings, and eventually was selected for several top leadership positions. However, to get the most of the program, you really need to join earlier than junior year to get promoted and get the opportunity to be selected for leadership positions. Those leadership positions and the leadership positions I obtained in Boy Scouts, right up to Eagle Scout were a tremendous experience for me and will help me to build my future on no matter where I go. Recently I received an LOA to the USNA, so I am guessing this all helped me, but it's the whole person view that they say is important.
Do it because you want to, not because you have to. I enjoyed, and still enjoy going to drills for the comradery and the activities, and the opportunity to volunteer. Joining at this late stage will not really help, but it will not hurt. The later you join, the less time you have to build up your leadership skills and be selected for leadership positions.
Good luck to everyone.