The UK government are no longer recommending mRNA booster doses for young people. They realized the risk-benefit analysis has dramatically changed.

NHS spring 2023 COVID-19 booster campaign eligible groups: adults aged 75 years and over.

Sadly in the US the CDC currently recommends one mRNA updated booster dose:
For [b]everyone aged 5 years[/b] and older who completed their primary series 💔
For [b]children aged 6 months[/b]–4 years who completed the Moderna primary series 💔