Development doesn't stop at birth. In fact, some of the most dramatic changes occur in the first years of life.

Neonatal Period (First 28 Days):

• Circulatory changes: The ductus arteriosus and foramen ovale close as the baby begins breathing air
• Thermoregulation: Newborns can't regulate temperature well initially
• Immune system: Dependent on maternal antibodies (IgG crosses placenta; IgA in breast milk)
• Brain: Rapid growth continues — brain reaches 90% of adult size by age 2

Infancy (1 month - 1 year):

• Weight triples in the first year
• Length increases by about 50%
• Motor development: Head control → rolling → sitting → crawling → walking
• Brain: Massive synaptic pruning and myelination

Childhood (1 year - Puberty):

• Steady growth (about 5-7 cm/year)
• Brain reaches adult size by about age 6
• Immune system matures
• Cognitive development accelerates

Growth Hormone: Throughout childhood, GH from the pituitary promotes growth of bones and tissues. Children with GH deficiency grow slowly and may be treated with synthetic GH.

Puberty is the developmental period when a child becomes capable of reproduction. It involves dramatic physical and hormonal changes driven by the reactivation of the HPG axis.

What Triggers Puberty?

During childhood, the HPG axis is suppressed (even though GnRH, FSH, LH, and sex hormones are present). As puberty approaches, the hypothalamus becomes less sensitive to negative feedback, allowing:

• Increased GnRH pulse frequency
• Rising FSH and LH levels
• Increasing sex hormone production

Changes in Females:

• Thelarche: Breast development (usually first sign, around age 10)
• Pubarche: Pubic hair growth (adrenal androgens)
• Growth spurt: Earlier than boys; about 6-10 cm/year at peak
• Menarche: First menstrual period (average age 12-13)
• Hips widen; body fat increases (especially hips, thighs, breasts)

Changes in Males:

• Testicular enlargement: First sign (around age 11-12)
• Pubarche: Pubic hair growth
• Growth spurt: Later than girls but more intense
• Spermatogenesis: Begins around age 13-14
• Voice deepens (larynx grows)
• Muscle mass increases dramatically
• Facial and body hair develop

Timing Variation: Normal puberty can begin anywhere from 8-13 in girls and 9-14 in boys. Early puberty (precocious) or delayed puberty may warrant medical evaluation.

After puberty, the body enters a long period of relative stability — adulthood. Growth stops (epiphyseal plates close), and the body focuses on maintenance rather than development.

Peak Function: Most physiological systems reach peak performance in the 20s:

• Cardiovascular: Maximum cardiac output, blood vessel elasticity
• Respiratory: Peak lung capacity
• Musculoskeletal: Maximum bone density (around age 25-30), muscle strength
• Reproductive: Peak fertility (especially in males; female fertility begins declining in late 20s)

Maintenance Phase:

• Homeostatic systems work to maintain stable internal conditions
• Tissues are continuously repaired and replaced (skin, gut lining, blood cells)
• Hormone levels remain relatively stable (with cyclical variations in women)

Lifestyle Impact: This is when lifestyle choices have major effects on long-term health:

• Exercise builds peak bone density and muscle mass
• Diet affects cardiovascular health, metabolism
• Smoking, alcohol, and other exposures cause cumulative damage

Reproductive Years: For women, fertility is highest in the 20s and declines significantly after 35. For men, fertility declines more gradually with age but sperm quality does decrease.

Aging is characterized by the gradual decline of physiological function. While we often think of aging as happening in "old age," many processes begin much earlier.

Cellular Aging:

• Telomere shortening: Chromosome tips shorten with each cell division; when too short, cells stop dividing
• Oxidative damage: Free radicals damage DNA, proteins, and lipids over time
• Cellular senescence: Old cells accumulate and secrete inflammatory factors

System Declines:

• Cardiovascular: Arteries stiffen, heart muscle thickens, maximum heart rate decreases
• Respiratory: Lung capacity decreases; chest wall stiffens
• Musculoskeletal: Bone mass decreases (osteoporosis risk); muscle mass decreases (sarcopenia)
• Nervous: Brain volume decreases; reaction time slows; memory changes
• Endocrine: Hormone production declines (growth hormone, testosterone, estrogen)
• Immune: Reduced response to new pathogens; increased inflammation

Menopause (Women): Average age 51. Ovarian follicles depleted; estrogen drops. Effects include hot flashes, bone loss, cardiovascular changes. Hormone replacement therapy can help but has risks.

Male Reproductive Ageing: Testosterone gradually declines (about 1% per year after age 30). There is no abrupt male equivalent of menopause, but some men experience reduced muscle mass, libido, and energy as they age.

Successful Aging: While aging is inevitable, lifestyle factors significantly affect how we age:

• Exercise preserves muscle, bone, and cardiovascular function
• Good nutrition supports cellular repair
• Social engagement and mental activity support brain health
• Avoiding smoking and excess alcohol reduces cumulative damage