It seems that this is different within the EU. Dutch passports are valid for:
10 years for persons aged 18 and over and 5 years for persons under 18.
So it seems that, even in the EU, every country has it's own rules!
For a passport of The Netherlands (which happens to be in the EU 😁) it is 10 years.
Source (in Dutch) https://www.rijksoverheid.nl/onderwerpen/paspoort-en-identiteitskaart/vraag-en-antwoord/duur-geldigheid-paspoort-of-nederlandse-identiteitskaart