I switched my boys (adopted at 2 months and 7 months, respectively, though they were about 3 weeks in age apart by the time they were together at my house) at about 8 months. Only because my 15 yr old senior was the only one who would eat the kitten food, the boys had no interest by that point. She'd eat their food and they ate hers so I figured it was time.
Both are now healthy, active and happy but 15 lbs a piece
The one adopted at 2 months was the runt of the litter, the other was big for his age when I adopted him but is pretty svelte considering he weighs so much, he carries the weight well since he's just a big meaty muscled cat. His brother not so much, he's a fluffball of fur and chub but vet says as long as they're as active as they are, it's all good.