The sacrament of confirmation leaves an indelible mark on your soul for eternity. It's about activating the grace you received at your baptism and becoming the person Christ created you to be. It is not so much about becoming an "adult in the Church" or deciding for yourself that you are going to be Catholic. Rather, it is when you are sealed with the Holy Spirit, and the graces you have received are stirred up in your heart so that you can live out Christ's mission in your life.
Jesus said, "Go, therefore, and make disciples of all nations” – Matthew 28:19