You will need to use a Credit Card to prove your 18 Years old and over to remove the Restriction on the MyO2 App. It can be removed in any O2 Store with Photo ID however they are closed just now.
How to remove o2 Age Restriction Content here - https://www.o2.co.uk/help/safety-and-security/age-restricted-content-and-age-verification
If you need to contact o2 you can find the details here - Guide: Coronavirus Community Help and Support
Advice from an O2 Guru regarding a way to get through:
When you call, you will be asked for the reason you are calling,
Stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to an advisor.
Press Upgrade, Lost/Stolen or Fraud if you have to.
8am is the best time to call