Final answer:
The individual described in the question is likely Edvard Grieg, a Norwegian composer, but the references provided do not relate to him or his work, as they focus on various visual artists and other composers.
Step-by-step explanation:
The individual in question who trained in Leipzig and later incorporated folk elements into his works by age 21 after studying the native music of Norway does not match any of the artists mentioned in the reference sections. Instead, the description is reminiscent of Edvard Grieg, a Norwegian composer and pianist who is indeed known for infusing his compositions with Norwegian folk melodies after studying at the Leipzig Conservatory.
However, none of the provided references, which discuss artists like Paul Klee, Emanuel Leutze, Stephen Foster, Georg Baselitz, and Caspar David Friedrich, align with the question about Norwegian folk elements. This discrepancy indicates that this particular schoolwork question might have been mixed with irrelevant reference information. The person inquired about is not covered in the provided materials, which focus on visual artists and composers that do not closely fit the description.