Final answer:
Concerns about darkness are common in toddlers and preschoolers, as this is a normal part of their development in understanding the world and dealing with fears of the unknown.
Step-by-step explanation:
The question asks about which age group often has concerns about sleeping in the dark. Concerns about darkness and the desire for a night light or some other source of comfort during the night are common in toddlers and preschoolers. These young children are developing their imaginations and are learning to distinguish between reality and fantasy, which can sometimes result in fears, including a fear of the dark. This is part of normal childhood development as they start understanding their surroundings and may face anxieties regarding the unknown which darkness often represents.