Hello, Kate!

This varies very much, depending in the size of the baby, the roominess of the carrycot/car seat, the season (fluffy/less fluffy clothes) and on the parents' preference, but I can tell you that, in general, it is around 6 months of age. It can be sooner (about 4-5 months) or later (7-10 months), if the bassinet is super large or the baby calm enough not to need a better view.

The only important thing is not to put your child sitting if the child didn't sit up by itself, without any help - I mean, you can even use the seat unit in the fully reclined, lie-flat position right from birth, if the baby is truly in the flat position.

I absolutely recommend reading this article I wrote about the topic of passing on to the pushchair setup/seat unit, it will talk about this problem much more in-depth.