An infant client is able to stand holding on to objects, plays "peek-a-boo," and is starting to say "mama" and "dada." Which age group does this behavior most likely correspond to?

The behaviors described align well with developmental milestones expected around 9 months of age. At this stage, many infants can pull themselves up to a standing position while holding onto furniture or other objects for support, demonstrating increased strength and balance. Furthermore, engaging in games like "peek-a-boo" indicates an understanding of object permanence, which is typically developed around this age.

In addition, starting to say "mama" and "dada" suggests the beginning of expressive language development, where the infant begins to associate sounds with familiar figures. These milestones are characteristic of cognitive and social-emotional growth that occurs around 9 months.

Younger infants, such as those around 5 or 6 months, are generally still developing basic motor skills and are not yet standing while holding on or engaging in interactive play like "peek-a-boo." Similarly, infants at 12 months have usually developed more advanced language skills and are often walking independently, which makes them a bit more advanced than the behaviors described in the question. Thus, the age group that best matches the reported behaviors is 9 months.