What you have to assume, unless told otherwise
No, I don't. I have an imagination, and I don't always turn it off just because I'm watching a fiction movie.
Given no information to the contrary, why would anyone assume Luke had been with them only what, one year? 2 years? 3 years?
Here's what I wrote earlier:
Heck, for all we know from ANH, Luke spent most of his childhood in the care of a larger, warmer family, and only started living in the isolation of the homestead when he became old enough to be useful there. Kinda like professional apprenticeships in the days before formal education of children became the norm - not as tough as slavery, but nowhere near as loving as parenting as we think of it today.
There's no assumption
there. It's an idea
, one neither founded in nor precluded by the the movie itself, nothing more.
The fact is, you
assumed he'd been with the Larses since birth, although there are all sorts of historical examples in which that would be abnormal. You're welcome to your own interpretation of what the movie doesn't tell us, but there's no need to scold others for working within the confines of the evidence to offer a different conclusion.