@fairynick
I’d like to suggest a reverse and ask people on this thread what they would say or think if their son turned around age 8, 10, or 13 and said “mum I’m a girl, I’m a trans girl”. I’m pretty sure many of you would say “you don’t know until you’re older, you’re too young enough to know”.
So you can know your gender as a child when you’re cis, but not when you’re trans. The transphobia on mumsnet is unreal.
There is nothing wrong with child being identified as a particular sex - sex is biological. Knowing you are a boy or a girl is no more damaging than knowing whether you have blue eyes or brown. It is a fact.
And yes - if my son said what you suggested at the ages you suggested, I probably would say "You won't know if you are trans until your hormones have settled down - and that won't be for a while yet." Almost everyone finds aspects of their sexuality awkward and confusing, whether they are straight, gay - whatever. I would also point out that they would be able to live how they wished, but that they can't change sex - because that is what they are.
BTW - for future posts: sex IS NOT the same as gender
and there is no such thing as a "cis" person. It is a very offensive term and I am sick of this biologicalsexphobia.