Final answer:
A child's brain is the most plastic due to the periods of blooming and pruning during early development, making it more adaptable compared to the brains of a teenager, an adult, or an elderly person.
Step-by-step explanation:
The concept of brain plasticity refers to the brain's ability to change and adapt as a result of experience. Brain plasticity is greatest in childhood and slowly decreases with age. Therefore, among the options provided, a child's brain would be the most plastic. This is because, during childhood, there is a significant period known as blooming, followed by pruning, where the size of the brain doesn't significantly increase, but the number of neural connections, efficiency, and functionality do. The brain continues to develop throughout adolescence, where processes like synaptic pruning and myelination become prevalent, making the adolescent brain also quite plastic, but less so than a child's.
The frontal lobes become fully developed by 25 years old, and this maturation involves the development of complex cognitive functions and decision-making abilities. Contrastingly, an adult's brain has less plasticity compared to an adolescent's, and an elderly person's brain would typically present even less plasticity due to age-related decline in cognitive functions and structural brain changes.