What's a good word to describe adults who are not yet parents?

"Adults".

The word "adult" does not imply that one is a parent, so there is no need for a word that describes a childless adult.

• childless (comes with the expectation of adult and married)

• barren (more about the inability to have children rather than the state of lack thereof)

Doing a wikicrawl revealed these two terms:

• DINK or DINKY — dual (or double) income, no kids (yet)
• childfree — individuals who neither have children nor desire to have children

Both of these hold a lot of connotation. DINK is somewhat derogatory; childfree is almost seen as a lifestyle choice similar to vegetarianism or recycling.