Your skin is the body's largest organ and the most visible record of how you've lived. Collagen — the structural protein that keeps skin firm and elastic — begins to decline at around 1% per year from age 25, accelerating with smoking, sun exposure and poor diet.
Collagen makes up about 75% of dry skin weight. Its gradual loss is the primary cause of wrinkles, sagging and the thinning of skin with age. The process is natural but highly modifiable — smokers lose collagen at nearly double the natural rate, while a diet rich in vitamin C, zinc and antioxidants actively supports its synthesis.
A study comparing identical twins where one smoked found the smoker looked on average 2.5 years older per decade of smoking. The mechanism is twofold: nicotine restricts blood vessels reducing nutrient delivery, and the chemicals in smoke directly degrade collagen and elastin.
You're born with all the hair follicles you'll ever have — approximately 100,000 on the scalp. Some follicles become dormant with age, while genetics, hormones and certain health conditions determine how many remain active. The 50-50-50 rule from JAMA suggests that 50% of people have 50% grey hair by age 50.
Tooth enamel cannot regenerate. Once worn away by acid erosion, abrasion or decay, it is lost permanently. The rate of enamel loss is heavily influenced by diet (acidic foods and drinks), oral hygiene and grinding habits.