Excerpt from Missionary Preparation Article, "When Should I Serve?" :
"When changes to the missionary age eligibility were announced, it is possible your “life plan” was altered and you’re no longer sure when you should go. When to serve a mission is an important decision.
How Will I Know When I'm Ready for a Mission?
Meeting with your bishop will help you know when you are ready to serve.Elder Nelson explained, “These age adjustments are new options now available to bishops in evaluating what is best for each of his youth. Young men or women should not begin their service before they are ready spiritually and temporally.” Many factors, such as school, health, worthiness, finances, and preparation, affect when you should serve. Prayerfully consider all of your options. Consistent prayer and regular meetings with your local priesthood leaders will help you know when you are sufficiently prepared."