Which of the following is a common sign of developmental delay in children?

Not meeting age-appropriate physical milestones is indeed a common sign of developmental delay in children. Developmental milestones are specific skills or tasks that most children can perform at certain ages, and they cover various domains, including physical, cognitive, social, and emotional aspects of development. When a child does not achieve these milestones within the expected time frames, it may indicate a delay in development.

For example, if a child is not able to crawl, walk, or engage in basic motor skills at the ages typically expected for those milestones, it raises concerns and suggests that the child may need further evaluation and support. Identifying these delays early can be crucial for providing appropriate interventions that can aid in the child's development.

Other options illustrate behaviors that fall within typical developmental progressions for children. Speaking in full sentences by age two is actually considered advanced speech development, while learning to ride a bicycle by age four and engaging in cooperative play with peers are skills that many children commonly develop around those ages, thereby indicating normal growth and development rather than delay.