What’s the difference between 젊다 and 어리다?

It may help to back up first and consider the different phases of life in Korean:

영아	유아	아동기	청소년기	성인기	중년기	노년기
infancy	toddlerhood	childhood	teenage	adulthood	middle age	old age

어리다 typically describes someone in the 아동기 phase.
젊다 typically describes someone in the early part of 성인기 (high school through 30s).