The UK telecommunications regulator, Ofcom, publishes some useful annual research on this topic [1]. According to the media use and attitudes report 2023 [2]:
> Smartphone ownership shifts markedly in [the children aged 8-11] group, which correlates with the children’s transition to secondary school … As reported by parents, more than half of 8-11-year-olds (55%) owned a mobile phone, a significant increase on children aged 3-4 and 5-7 (both at 20%).
> Smartphone ownership shifts markedly in [the children aged 8-11] group, which correlates with the children’s transition to secondary school … As reported by parents, more than half of 8-11-year-olds (55%) owned a mobile phone, a significant increase on children aged 3-4 and 5-7 (both at 20%).
[1]: https://www.ofcom.org.uk/research-and-data/media-literacy-re...
[2]: https://www.ofcom.org.uk/__data/assets/pdf_file/0027/255852/...