Whatever martial arts you do, a rigorous work on suppleness is essential to acquire ease in your movements and prevent injuries. In taekwondo, being supple in the hips and in the legs is a decisive asset, even more so since the points gained by hitting the head have been increased.
Oh I can already hear the adult practitioners saying: “this is much harder after teenage development”. Correct. Joints flexibility is superior till teenage-hood. Yet, everything is possible if you train with the correct physical positions, without overdoing it, and with much, much discipline! Keep in mind that it will definitely be harder for those who did sports which training included a lot of leg muscle-development exercises.
If you work hard at home to enhance your flexibility, here is a must-see video presenting essential stretching exercises: