As a teacher, I will not give you direct answers that will complete your school assignment for you. I will, however, offer some hints that might prompt your own ideas. Then, you can flip through your book for the page numbers of specific examples.
Think about the overall vitality of Phineas, his carefree and adventurous nature, his vibrant energy and athleticism. Are they not youthful aspects? Consider how he breaks rules and ignores authority. Isn't he boisterous and spontaneous? Are these qualities generally associated with adulthood?
I have faith in you, Tzivia. If you've read A Separate Peace, you'll be able to come up with the information expected of you.