The US and Canada since 2018. The EU since 2022 (for new models: existing models get until 2024)[1].
As for why in the US: literally too many kids being killed by their parents in SUVs and trucks [2].
[1] https://bmdv.bund.de/SharedDocs/EN/Articles/StV/Roadtraffic/...
[2] https://www.congress.gov/congressional-report/110th-congress...