You're the second person today with this issue so O2 must have changed something.
Check your age restriction settings Guide: O2 Age restriction
and youtube advice here https://support.google.com/youtube/answer/174084?co=GENIE.Platform%3DAndroid&hl=en
It will be added as credit to your balance.
Yes it will.
You have paid O2 £1.00 to verify that the card is yours and thus over 18.
They then give it back to you as credit.
Track usage, upgrade and get support with the My O2 appDownload My O2