The author probably means that people in their 30s will be the last generation to have witnessed telephones plugged into walls — i.e., the generations after the

The author probably means that people in their 30s will be the last generation to have witnessed telephones plugged into walls — i.e., the generations after them, people in their 20s and younger, won't have. That anyone in their 40s, 50s, 60s, etc. has witnessed and used telephones plugged into walls is implied.