I remember there was a case a few years ago where Stonewall said they believed very young children could tell they were transgender.
In looking for that, I found things like:
https://rethinkingschools.org/articles/can-a-4-year-old-know-her-gender-identity-yes/
https://cafemom.com/parenting/213965-mom-inspiring-story-transitioning-son-daughter
https://unherd.com/newsroom/why-are-we-celebrating-a-four-year-old-transitioning/
https://www.businessinsider.com/how-my-toddler-came-out-as-trans-to-our-family-2021-9?op=1
https://archive.ph/vuZGw
https://www.nbcnews.com/storyline/transgender-kids/jacob-s-journey-life-transgender-5-year-old-n345131
This is just from the first page. I didn't go any further. There are so many stories published about very young children being transitioned.
Anyway, this is what Stonewall said in 2022.
https://reduxx.info/stonewall-says-toddlers-can-recognize-they-are-transgender/
The tweet from Stonewall 22nd July 2022:
"Research suggests that children as young as 2 recognise their trans identity. Yet, many nurseries and schools teach a binary understanding of pre-assigned gender.
LGBTQ-inclusive and affirming education is crucial for the wellbeing of all young people!"
And the tweet was in response to this metro article:
https://metro.co.uk/2022/07/19/my-4-year-old-is-gender-nonconforming-her-nursery-doesnt-respect-that-16969054/
So, for those who doubt that that 4 year olds are being transitioned, I suggest you might like to do your own search.
There is also one particular young male child who has parents who have regularly posted photos on twitter over the past few years documenting that child's transition, If I remember correctly, I most recently saw they were protesting last year that their child was being excluded from female school sports. But, I cannot remember the child's name and there are now so many search results on twitter that I cannot find those specific parents.