Answer: Youngest = Marcus
Oldest = Cynthia
Explanation:
Molly is older than Jenna, but younger than Brett:
Jenna < Molly < Brett
Brett is younger than Cynthia, but older than Jenna:
Jenna < Molly < Brett < Cynthia
Jenna is older than Tina and Marcus:
Tina & Marcus < Jenna < Molly < Brett < Cynthia
Marcus is younger than Cynthia and Tina:
Marcus < Tina < Jenna < Molly < Brett < Cynthia