A Human Life: The Body from Conception to Old Age Timeline

A Human Life: The Body from Conception to Old Age 26

Before birth Fertilization Human body over a lifetime A sperm cell and an egg fuse into a single cell, about 38 weeks before birth.
Before birth The heart starts beating Human body over a lifetime Three weeks after fertilisation, the first organ to work.
Before birth Embryo becomes a fetus Human body over a lifetime After eight weeks all major organs are in place.
Age 0 Birth Human body over a lifetime About 50 cm long and 3.5 kg. The lungs take their first breath.
Age 0 About 300 bones Human body over a lifetime A newborn has around 300 bones and cartilage pieces. Many fuse, leaving 206 in an adult.
Age ½ First tooth Human body over a lifetime Usually a lower front tooth, between 6 and 10 months.
Age 1 First steps Human body over a lifetime Most children walk alone around their first birthday.
Age 2 Half of adult height Human body over a lifetime A two-year-old is roughly half as tall as it will be as an adult.
Age 3 All 20 milk teeth Human body over a lifetime The set of milk teeth is complete by about 33 months.
Age 6 First permanent teeth Human body over a lifetime The six-year molars and the first adult front teeth come through.
Age 6 The brain at 90% of adult size Human body over a lifetime Most brain growth happens before school age. The rest of childhood is about wiring.
Age 11–13 Growth spurt Human body over a lifetime The fastest growth since infancy: at about 11 in girls and 13 in boys.
Age 12½ Puberty: first period Human body over a lifetime Menarche at 12½ on average. Boys reach the equivalent step, spermarche, at about 13.
Age 12–13 Last milk tooth falls out Human body over a lifetime By about 12 or 13 the milk teeth are all replaced.
Age 15–17 Adult height reached Human body over a lifetime Growth plates close: at about 15½ in girls and 17 in boys.
Age 17–21 Wisdom teeth Human body over a lifetime The third molars come through, between 17 and 21, if at all.
Age 25 The brain is fully mature Human body over a lifetime The prefrontal cortex, for planning and self-control, is the last part to mature, in the mid-twenties.
Age 25 The last bone stops growing Human body over a lifetime The inner end of the collarbone is the last growth plate in the body to close.
Age 20–25 Lung function peaks Human body over a lifetime Breathing capacity is at its highest in the early to mid-twenties, then slowly declines.
Age 30 Peak bone mass and muscle mass Human body over a lifetime Bones are at their densest and muscles at their largest. From here muscle mass slowly declines.
Age 35 Fertility declines Human body over a lifetime Female fertility starts falling in the early thirties, faster after 35.
Age 40 Height begins to shrink Human body over a lifetime Discs between the vertebrae compress. People lose about a centimetre per decade, more after 70.
Age 40–50 Reading glasses: presbyopia Human body over a lifetime The lens of the eye stiffens and close focus fades. Almost everyone needs reading glasses by 50.
Age 51 Menopause Human body over a lifetime Periods stop, at 51 on average. Bone loss speeds up in the years that follow.
Age 65 Age-related hearing loss Human body over a lifetime High tones fade first. About a third of people between 65 and 75 have hearing loss.
Age 73 Average life expectancy Human body over a lifetime Life expectancy at birth worldwide is about 73 years, varying from the low 50s to the mid 80s by country.