I like to use this at the beginning of the year or the beginning of the semester. It is important for students to set goals for themselves and when it comes to teenagers, their likes, dislikes, goals, etc. change frequently.
I have them write these letters to themselves not only to see their writing skills at the beginning of the year/semester but also to get to know them as well.
When I pass these letters back to them at the end of the year or end of the semester, the students find it very interesting to see what they wrote 6 months or a year earlier and what their interests and goals were at that time, if they reached their goals, or if their goals changed.
This is easily able to be edited by the teacher to include topics the teacher would like covered at that time. Teachers can make the topics as specific or broad as they wish.