Final answer:
Franz Schubert was 19 years old when he wrote the famous lieder 'Erlkönig' during the Classical period of music.
Step-by-step explanation:
Franz Schubert was 19 years old when he wrote the famous lieder 'Erlkönig'. Schubert composed this masterpiece in 1815, during the Classical period of music. 'Erlkönig' is based on a poem by Johann Wolfgang von Goethe and tells the haunting story of a father and his son being pursued by the mythical Erlking in a horseback ride through the night.