young
This book will definitely appeal to teenagers and young adults.

single
The number of single-adult households has doubled in the past 30 years.

consenting
What consenting adults do in private is their own business.

responsible
I simply can't believe that responsible adults allowed a child to wander the streets.

education, literacy
population

Adult is used with these nouns:
bird, child, conversation, daughter, diaper, education, entertainment, fare, fiction, form, illiteracy, incest, learner, life, literacy, male, mortality, novel, offender, patient, population, reader, responsibility, site, size, son, specimen, stage, supervision, ticket, tooth, video, viewer, woman