Complex sentence: At 15 until the age of 18 he attended the Phillips Academy, in Andover, Massachusetts; where he studied painting with Bartlett Hayes and won the Samuel F. B. Morse Prize for most promising art student.
Split into simpler sentences: At 15 until the age of 18 he attended the Phillips Academy, in Andover, Massachusetts; where he studied painting with Bartlett Hayes. At the Academy he won the Samuel F. B. Morse Prize for most promising art student.