Start with the eyes, ears and cord
The youngest kittens are unmistakable once you know the signs. A kitten in her very first days has eyes sealed shut, ears folded flat against her head, and often a remnant of the umbilical cord still attached, which dries and falls away early on. Eyes begin to open during roughly the second week, appearing as slits at first and often looking blue-grey, and the ears gradually unfold and stand over the following weeks. A kitten whose eyes are fully open and following movement, with ears up and alert, is past the newborn stage even if she is still tiny, and a kitten whose eye colour is clearly changing from the early blue is older again. Handle a found kitten minimally and keep her warm while you assess her, because warmth matters more than a precise age in the first hour.
Movement tells you as much as appearance
How a kitten moves is a reliable age ladder. Newborns crawl on their bellies, paddling weakly and unable to support their weight. Over the next stage the legs begin to hold her for shaky standing and a few staggering steps, usually with the tail up for balance and frequent tumbles. A kitten who walks with reasonable steadiness, begins to play with swatting motions, and can climb out of a shallow box is further along, and one who runs, pounces and grooms herself confidently is older still. Toileting sits on the same ladder: the youngest kittens need their mother, or a carer, to stimulate them to pass urine and stools, while an older kitten who uses a tray independently has passed that stage entirely.
Teeth and eating refine the estimate
If you can look gently in her mouth without distressing her, teeth narrow the age considerably. A mouth with no teeth visible belongs to the earliest weeks; small, sharp front teeth arriving signal the next stage, with the longer fang-like teeth and back teeth following in sequence, and a kitten chewing rather than merely lapping is showing you both teeth and readiness together. What she can eat matches: the youngest cannot lap at all and need to be fed as newborns, a middle-stage kitten laps milk or gruel messily, and an older kitten eats soft solid food with enthusiasm. Weight adds a cross-check, since healthy kittens gain steadily from a very small starting point, but breed and nutrition make weight the least precise clue on its own.
Use the age to set tonight's care, then get it confirmed
The point of the estimate is practical: it tells you how warm she must be kept, how she needs feeding, whether she needs help to toilet, and how urgent veterinary input is, with the youngest found kittens needing help the same day rather than the same week. Write down the clues you used and the age you settled on, because that reasoning helps the veterinarian and any rescue you consult. Seek advice immediately if a found kitten is cold, limp, injured, has fly eggs or wounds, or is not feeding, whatever age you judge her to be, since found kittens can deteriorate quickly and an estimate is never a substitute for an examination.
Her first record starts with your best estimate
A found kitten arrives with no history, so the record you create on day one, estimated age, weight, where she was found, and what she could do, becomes the foundation of everything that follows. KinsLe Pet Passport keeps that first record together with the records she accumulates from then on, in one portable packet that can travel with her to the veterinarian, a rescue or her eventual permanent home.