Cats compress a human's first quarter-century into two years, then settle into a steady, dignified four-human-years-per-year pace. That's the whole model — 15, +9, then +4 each year — and it's the one feline veterinary bodies use for life-stage guidance.
The math in action
A 1-year-old cat ≈ 15 human years: adolescent, fertile, opinionated. Two years ≈ 24 — fully adult. From there add 4 per year: a 5-year-old is ~36, a 10-year-old is ~56, and a 20-year-old great-aunt of a cat sits near 96 human years. Unlike dogs, size barely matters — a Maine Coon and a Siamese age on essentially the same curve.
Life stages and what they mean
The AAFP/AAHA staging this calculator reports: kitten (to 1), young adult (1–3), prime (3–7), mature (7–11), senior (11–15), and super senior beyond. The practical uses: mature cats benefit from annual bloodwork, seniors from twice-yearly checks — age-in-human-terms is a good intuition pump for why.
Indoor vs outdoor: same clock, different risks
Biology ages both the same; environment doesn't. Outdoor cats average dramatically shorter lives (traffic, predators, disease) — often 2–5 years versus 12–18 indoors. The conversion holds either way; the actuarial table doesn't.
Records and the dog comparison
Creme Puff of Austin, Texas reached 38 — roughly 168 human years — which remains the verified record. Dogs age on a steeper, size-dependent curve; if the household has both species, the dog age calculator shows why the cat usually wins the longevity race. For your own milestones, the age calculator counts human years precisely.
Convention-based estimate; individual health and genetics vary.