The Irish communications regulator, Comreg, determines categorisation for what is considered "adult" content on YouTube (well, or at least sets guidance, which YouTube somewhat clumsily implements, but the guidance is vague and crap). The Irish regulator is allowed to do it Europe wide, as under the Single Market YouTube's European operations are based in Ireland for tax reasons.
The AVMS and DSA are going to force age gates in Europe to an unprecedented scale, as will the Online Safety Bill in the UK.
Citation needed?
What constitutes adult content on youtube?
Other than for actual online gambling I struggle to see age gates are a thing anywhere in the EU.