We typically recommend LTW Level 1 for students 7th grade and up. Students younger than that typically are not prepared for the deeper level of thinking towards which LTW guides them. Students, regardless of age, should NOT begin level 2 until they have completed level 1 for it lays the key foundations upon which all other writing is built.
Middle school students who have experienced basic writing exercises and who know the elements of grammar are prepared for this program.
The practiced teacher can use the ideas contained in this curriculum for children at any age level, and The Lost Tools of Writing™ has been used as early as third grade. If you have older students (juniors and seniors) who are experienced writers you could easily tailor it to them and go through it at a faster pace.