The Short Answer: Around Three Weeks Old
Young guinea pigs need separating by sex at roughly three weeks of age, far earlier than most new owners expect. Male pups mature sexually very young, and a boar pup left in with his mother and sisters beyond that window can mate successfully, leaving you with pregnant females who are themselves barely weaned. Female pups also mature early, though the urgent driver of the three-week deadline is the males. Plan the calendar backwards from the birth date the day the litter arrives, not when the pups start looking grown, because a chunky three-week-old pup still looks like a baby while being entirely capable of starting the next litter.
Sexing Pups Accurately Is the Hard Part
The deadline is easy; knowing who is who is not. Sexing a tiny guinea pig involves gentle pressure just above the genitals and learning the difference in shape between boars and sows, and even experienced keepers re-check pups more than once as they grow, because early calls can be wrong. Build two checks into the routine, one in the second week and a confirming check on separation day, and have any genuinely doubtful pup examined by a vet or an experienced rescue volunteer before you trust the split. A single mis-sexed boar left in the girls' cage undoes the entire exercise, so treat separation day as provisional until the confirming check a week later agrees with it.
How to Arrange Separation Day
Weaning and separating land at roughly the same moment, so organise the housing before the day itself: a cage for the young boars, a cage for the young sows, who usually stay with their mother, and a plan for the adult boar, if there is one, who must not be returned to the mother either. Keep brothers together and sisters together, because guinea pigs are social animals and a pup moved alone into a solitary cage pays for your organisation with loneliness at exactly the age he or she is also losing the nest. Move the pups with their familiar bedding and a hide from the original cage, keep the new cages in the same room so sounds and smells stay familiar, and handle everyone briefly and calmly rather than making the day a series of chases.
Feeding and Watching in the First Week After
Separated pups should already be eating hay, pellets and small amounts of vegetables confidently, since guinea pigs begin sampling solid food within days of birth. Weigh each pup every few days across the transition and keep a simple written record: steady gain is reassuring, while a pup who stalls or loses weight after the move needs a closer look, a vet conversation if it continues, and possibly more time eating alongside mum under supervision if he was separated on the early side. Watch that the young boars' cage dynamics settle; brothers usually do well, but mounting and chasing in the first days should calm rather than escalate into bites.
If You Missed the Window
If pups are already older than three weeks and still mixed, separate them the day you realise, calmly and without blame, and make a note of the dates. Then watch the females over the following weeks for signs of pregnancy such as steady weight gain and a widening shape, and talk to your vet early so any pregnancy is monitored rather than discovered at delivery. Do not wait for certainty before splitting the sexes in the hope that nothing happened; every additional mixed day adds risk, and a vet can help you plan for either outcome.
Give the Litter's Jobs to Named People
Birth-date counting, two rounds of sexing checks, cage preparation and post-separation weighing are several small jobs that all fail if everyone assumes someone else did them. KinsLe's Care Teams lets you assign each of those litter tasks to a named family member or helper, with completed checks marked off so the three-week deadline never slips by unnoticed.