Which statement is not true concerning the brain's mass related to age? From early adulthood onward, brain mass declines. As one ages, the number of neurons decreases but the number of synaptic connections increases. The brain growth during the first few years of life is due mainly to an increase in the size of neurons already presen During the first few years of life, there is continued myelination of axons which contributes to brain growth.

1. We first consider the statement, "From early adulthood onward, brain mass declines."  It is well established that after early adulthood, the brain undergoes gradual atrophy, including reductions in white matter and synaptic density. Therefore, this statement is true. 2. Next, we examine the statement, "As one ages, the number of neurons decreases but the number of synaptic connections increases."  Although some neuronal loss may occur during aging, evidence shows that there is also a decline in synaptic connections rather than an increase. Because aging generally results in a reduction in both the number and quality of synaptic interactions, this statement is not true. 3. The third statement, "The brain growth during the first few years of life is due mainly to an increase in the size of neurons already present," requires understanding how brain mass increases in early life.  In infancy, brain growth is predominantly driven by the establishment of new synaptic connections, increased dendritic arborization, and the addition of glial cells. While increases in neuronal size do occur, they are not the primary factor in overall brain growth. However, the statement is not as clearly incorrect as the one regarding synaptic connections during aging. 4. Finally, the statement, "During the first few years of life, there is continued myelination of axons which contributes to brain growth," aligns with well‐documented observations.  Myelination significantly increases during early development and contributes substantially to brain mass. Thus, the statement that is not true is: - \( \text{"As one ages, the number of neurons decreases but the number of synaptic connections increases."} \)