I started flushing when I was 18 and kept going and always got stones out. I would say that it would probably be beneficial to just keep flsuhing every so often, depending on your health, diet and how you feel. Starting at your age will bring you many benefits in the future, combined with other aspects of clean living (especially colon cleansing).
By the time you are 30 (my age) you will be leaps and bounds ahead of your contemporaries in health and life energy!
Don't assume you are stone free yet... and you can always do as MH suggests in the Barefoot Herbalist forum, to take Olive oil and lecithin every night for the rest of your life (a tablespon or so) to keep the liver and GB cleaning themselves continuously. But personally I would still flush occassionally even though doing this (at least twice a year, even if in perfect health).