Final answer:
Infants who begin to walk by themselves by around 12 to 15 months are known as toddlers. This term specifically reflects their development stage, characterized by improving motor skills and increasing independence in movement.
Step-by-step explanation:
By 12 to 15 months or so, infants typically start to walk by themselves, which is why they earn the name toddlers. At this stage, they exhibit considerable growth in their motor skills. They can pull themselves up to stand, walk with help, and even begin to venture steps on their own. By the age of two, toddlers are more adept at walking, can navigate stairs with assistance, and demonstrate increased coordination.
It's important to note that infants are termed crawlers before they start walking, as crawling is one of the first modes of movement they develop. Once they begin to walk independently, they transition from crawlers to toddlers. The term walkers may describe infants in the process of learning to walk, but it is not the official term used to denote this developmental stage. The term explorers can describe toddlers' behavior, as they often explore their environment, but it is not the specific term for this age group.