Approve what kids buy with Ask to Buy - Apple Support
"Who can use Ask to Buy
Families can use Ask to Buy after they set up Family Sharing. The family organizer can turn on Ask to Buy for any family member who isn't an adult. It's on by default for any children under 13. You'll be asked to set up Ask to Buy when you invite anyone under 18 to your family group.1
If a family member turns 18 and the family organizer turns off Ask to Buy, the family organizer can't turn it on again."
But you're right, at least according to Google Translate:
Godkänn vad barnen köper med Be om köp - Apple-support (SE)
"Om en familjemedlem fyller 18 och familjesamordnaren stänger av Be om köp går det att aktivera igen."
--> "If a family member turns 18 and the family coordinator turns off Ask to buy, it can be reactivated."
There is either some confusion with translation or there is a radical difference in policy. I would call Apple in your country and ask.
Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone ➞ Contact Apple for support and service - Apple Support
I also speak French and German and those versions are consistent with the US English version saying you cannot turn it on again. I am inclined to think that if you find you cannot turn it on again then the versions saying you cannot are correct (I don't speak Swedish).