That 'child' could be an older preschooler or a toddler or even a baby right down to almost newborn.
For most adult babies, their regression is limited and more under control, but for some, it is so significant and long-lasting that it creates a permanent change in the relationship paradigm.
This book has case studies on other couples that have embraced - willingly or not - varying aspects of the parent/child relationship. She then gives some clues and advice on taking some of the angst out of relationships where one is an adult baby and the other is struggling to handle it.