How old is a 21-month-old kitten in human years?
≈ 22 human years.
Approaching the prime adult years, physically at their peak.
Kittens front-load their ageing: the first twelve months are worth about 15 human years, which is why a six-month-old kitten is closer to a human pre-teen than a toddler.
Nearby ages
| 12 months | 15 human years |
|---|---|
| 18 months | 20 human years |
| 21 months | 22 human years |