How the human brain transforms from birth to old age

The human brain undergoes remarkable changes from conception to old age. Its development starts just 16 days after fertilisation, with rapid growth continuing through childhood and beyond. At birth, an infant’s brain weighs around 350 to 400 grams. It already contains nearly all the neurons it will ever have. By age six, it reaches about 90% of its adult size.

During middle childhood, synaptic pruning speeds up. This process refines the brain’s networks, improving efficiency. Peak cognitive processing speed is typically achieved between 16 and 25.

After 60, the brain loses roughly 0.5 to 1% of its volume each year. However, vocabulary and verbal reasoning can keep improving into the late 60s and early 70s. Adults who stay physically, socially, and intellectually active experience slower cognitive decline.

A cognitive reserve built over a lifetime may delay dementia symptoms by up to a decade. Yet by 85, about one in three people will still develop some form of dementia. Brain development is a lifelong journey. Early growth is rapid, while later years bring gradual decline. Active lifestyles and cognitive reserve can help preserve function, but age-related risks remain.