Judge the move by size and confidence, not the calendar
A young betta is ready for the main tank when it passes three practical tests rather than when it reaches a particular age. It must be too big for any adult mouth in that tank, it must feed boldly instead of waiting for scraps, and it must swim strongly enough to cross open water without exhausting itself. Growth rates vary widely between spawns and even between siblings, because the boldest youngsters eat the most. A calendar date moves the whole batch together; the tests move each young betta when that individual fish is ready, which is the difference between a smooth introduction and a fish that disappears in the first week.
The one rule that has no exceptions
Sex overrides every other consideration when young bettas move in with adults. An adult male betta defends his territory against any male, and a maturing young male placed in that tank will be attacked as soon as the adult reads him as a rival, which can happen before the youngster looks fully male to you. Young males are therefore never added to an adult male's tank, at any size. They are also the fish to separate earliest in a grow-out tank, since juvenile males begin sparring with each other as they mature and a youngster that is suddenly lingering at the surface with nipped fins is usually one that needed its own space a week earlier.
What changes when the tank holds females
A main tank of adult females, a sorority, accepts a young female betta more readily than a male's tank accepts anything, but the introduction still needs managing. The young female should be large enough that the adults cannot treat her as food, and the tank should be rearranged or planted densely enough that she is not the single new object of attention in open water. Established females have a hierarchy, and a young betta entering it will be tested; brief chasing is normal, while persistent pursuit, cornering, or torn fins mean she comes back out to grow on. Adding her at feeding time, with the lights lowered, spreads the adults' attention at the moment she appears.
Prepare the grow-out graduate properly
A young betta moving tanks does better when the two waters match in temperature and chemistry, because bettas feel a sudden change more than their hardy reputation suggests. Float and gradually mix water as you would for a new purchase, and time the move for a calm part of the day when you can watch the first hours rather than releasing the fish and leaving. Feed the main tank just before the introduction. For the first days, check specifically that the young betta is reaching food: adults that have fed in that tank for months know the routine, and a newcomer can miss meals while appearing settled, losing condition quietly behind the plants.
Reading the first week honestly
The first week after the move is an observation exercise. A young betta that explores, flares briefly, and returns to feeding is settling. One that hides continuously, hangs at the surface, breathes fast, or shows fresh fin damage each morning is not, and the kind response is a separate tank rather than more time to toughen up. Bettas do not toughen up; they lose ground. Keeping a brief daily note of appetite and fin condition for that first week turns a vague worry into a clear decision, and gives an aquatic veterinarian useful history if the fish later looks unwell.
Assigning the watch after the move
Because the deciding checks are daily and specific, they suit being shared rather than left to whoever happens to pass the tank. Setting the post-move watches as duties in KinsLe Care Teams, assigned per pet and stamped complete with a time, means the young betta's appetite, fins, and confidence are reviewed every day of that critical first week by a named person.