Age Calculator

Leap day babies (leaplings) born February 29 legally age on February 28 in non-leap years (or March 1 in some jurisdictions). February 29 only occurs in leap years (years divisible by 4, except centuries unless divisible by 400: 2000 was leap, 1900 wasn't, 2024 is, 2100 won't be). Born February 29, 2000: birthdays on February 29 in 2004, 2008, 2012, 2016, 2020, 2024 (leap years). In non-leap years (2001, 2002, 2003, 2005-2007, etc.), legally turn one year older on February 28 for most legal purposes (driver's license eligibility, voting, alcohol purchase). Example: born February 29, 2004, turning 21 in 2025 (non-leap year), legal drinking age reached February 28, 2025 (day before March 1), not March 1. Some jurisdictions specify March 1 as legal birthday in non-leap years—verify local laws. Age calculation: born February 29, 2000, today December 16, 2024 = 24 years, 9 months, 17 days (using February 29, 2024 as last birthday—leap year). Fun: leap day babies have "real birthday" every 4 years—some celebrate February 28, some March 1, some both, or wait for actual February 29. Only ~5 million people worldwide born on February 29 (~1 in 1,461 people). Famous leaplings: motivational speaker Tony Robbins, rapper Ja Rule, composer Gioachino Rossini. Leap day clubs exist (Honor Society of Leap Year Day Babies). Statistical quirk: probability of being born February 29 = 1/1,461 (365.25 days per year average).

Chronological age is time since birth (what age calculators show); biological age is physical condition of body relative to chronological age (health/aging biomarkers). Chronological age: person born January 1, 1980, today December 16, 2024 = exactly 44 years, 11 months, 15 days old (fixed, objective measure based on calendar). Biological age: assessed through: telomere length (DNA aging markers), methylation patterns (epigenetic clocks), cardiovascular fitness, grip strength, blood biomarkers (inflammation, cholesterol, glucose, hormones), cognitive function, skin elasticity, and other health metrics. Someone chronologically 50 might have biological age of 40 (excellent health, active lifestyle, good genetics) or 60 (poor health, chronic diseases, lifestyle factors). Factors affecting biological age: genetics (~25% of aging variance), lifestyle (diet, exercise, sleep, stress management), environment (pollution, toxins), medical conditions, and behaviors (smoking, alcohol). Tests estimating biological age: blood tests (GlycanAge, TruAge), DNA methylation tests (Horvath clock, GrimAge), fitness assessments. Implications: biological age better predicts health outcomes and mortality than chronological age. Two 60-year-olds may have vastly different health status and life expectancy. Interventions to reduce biological age: exercise (especially strength training and cardio), caloric restriction, quality sleep, stress reduction, avoiding smoking/excessive alcohol, social connections. Chronological age useful for: legal purposes, demographic data, social norms. Biological age useful for: health assessment, intervention planning, longevity optimization. Cannot change chronological age; can potentially improve biological age through lifestyle.

Infants age typically expressed in weeks (0-12 weeks), then months (0-24 months), then years. Age in weeks: born September 16, 2024, today December 16, 2024 = exactly 13 weeks old (3 months). Calculation: count complete weeks—September 16-23 = week 1, September 24-October 1 = week 2, ..., December 9-16 = week 13. Doctors often ask "how many weeks" for newborns—precise tracking important for developmental milestones, vaccine schedules, feeding changes. Pediatric growth charts use weeks initially. Age in months: After ~3 months, switch to months. Born September 16, 2024, today December 16, 2024 = 3 months, 0 days (exactly 3 months if month length allows). Used until age 2 (24 months) because development happens rapidly—difference between 18-month and 21-month toddler is significant. After 24 months, typically switch to years ("2 years old" vs "24 months old"). Some contexts continue months longer (daycare often uses months through age 3). Developmental milestones tracked: weeks for newborn (should regain birth weight by 2 weeks, smile by 6-8 weeks), months for infant/toddler (roll over 4-6 months, sit 6-8 months, walk 12-18 months, talk 18-24 months). Precision matters: preemie babies use corrected age (subtract weeks born early—born 8 weeks early, chronological age 6 months = 4 months corrected age for development). Feeding schedules: newborns every 2-3 hours, older infants every 3-4 hours, start solids around 6 months. Age calculator modes: toggle "show in weeks" for 0-3 months, "show in months" for 0-2 years, "show in years" for 2+. Context determines appropriate unit.

Time zones, exact time of birth, and calculation timing can cause apparent discrepancies near birthdays. Example: born December 16, 1990, at 11:30 PM. Checking age December 16, 2024 at 10:00 AM. Some calculators: "33 years, 11 months, 30 days" (haven't reached exact moment of birth yet—still technically 33). Others: "34 years" (birthday is today, count as 34). Most calculators use date only (ignore time): birthday = entire day, so anywhere on December 16, 2024 = turned 34. Time zone issues: born in London at 11:00 PM GMT December 16, 1990 = already December 17 in Tokyo (11 hours ahead). Person could be "34" in London but still "33" in some other time zone on December 16 (haven't reached December 17 yet). International Date Line complications. Calculator settings: "include birth date" toggle—some calculators let you choose whether birth date counts as day 0 or day 1 of life (affects "days old" calculation by 1). Medical contexts use exact hour (gestational age, premature birth), legal contexts typically use date only (don't need birth time for age verification). Cultural differences: some cultures count differently (Korean age system adds 1 year at birth and 1 year each New Year, so "age" can be 1-2 years higher than Western calculation). Recommendation: for legal/official purposes, use date-only calculation (ignore time of birth). For medical precision (newborns especially), include time. For casual use, date is sufficient. Discrepancies rare—typically only matter on actual birthday when checking age at specific time before exact birth hour.

Simplified conversions (multiply by 7 for dogs) are inaccurate; modern formulas account for non-linear aging but remain rough estimates. Old rule: dog age × 7 = human equivalent (5-year dog = 35 human years). Problems: dogs reach sexual maturity ~1 year (equivalent to teenager, not 7-year child), dogs don't live to 140 (20 dog years × 7), aging rate isn't constant (dogs age faster when young, slower later), and size matters significantly (small dogs live longer, age slower than large breeds). Modern approach: First year = ~15 human years (rapid growth and maturation), second year = ~9 human years (total 24 by age 2), subsequent years = 4-5 human years each (varies by size—small breeds ~4, large breeds ~6). Formula example: Human age = 16 × ln(dog age) + 31 (natural logarithm formula based on DNA methylation research). For 5-year medium dog: 16 × ln(5) + 31 ≈ 57 human years. Cats: Similar to medium dogs—first year ~15, second year ~9, then ~4 per year. 10-year cat ≈ 56 human years. Size effect in dogs: Chihuahua living to 18 is common (~90 human years). Great Dane living to 10 is old age (~70+ human years). Giant breeds age faster—biology, not just size (metabolic rate, growth hormone levels, earlier onset of age-related diseases). Limitations: Individual variation is huge (genetics, health, care quality), different organs age at different rates, and human equivalent is conceptual tool, not biological fact. Use conversions for: general understanding of pet life stage (puppy/kitten, adult, senior), setting expectations for care needs, explaining to children, and contextualizinglife span. Not useful for: specific medical decisions (use actual age and species-specific standards), precise health comparisons, or determining care based on "human age equivalent". Veterinary care based on actual pet age and health status, not conversion.