I started with descant in primary school, no idea what age, and got a tenor when I was 9 or 10. At secondary school I got individual lessons, and a treble not long after. I don’t find changing hard, I think for me where my hands are tells my body and brain what size instrument it is and the rest happens largely automatically.
Interestingly, bass is where I fall apart but great bass I am totally fine with. I have no recollection of learning to read music or the basics of a descant, so my theory is that C-instrument in treble clef is my default, really well ingrained in my brain. I can manage one step away, so F-instrument in treble clef, or C-instrument in bass clef. But make it the two steps to F-instrument in bass clef and I don’t do well at all!