What an absolute farce. We now have to verify our age when we log into My Community - All we have to do I tick that we are over 18 - not sure if anyone under 18 ticks it though. I have been involved in all this GDPR hassle so know what everyone needs to do about data protection. I don't think this involves asking for age consent to put a post anywhere, but I may be wrong!. Anyway, this is just a rant.
I know exactly what you mean. It's a one-off assertion that the member isn't under 18. The age 18 requirement is a policy decision by TalkTalk. Whilst Community is intended as a family-friendly forum the content isn't targeted at under 18's.
The GDPR requires parental consent for processing data of those aged under 16 so by ticking the not under 18 box TalkTalk can assert they have carried out what's needed for GDPR compliance and of course don't have the extra overhead of gaining parental consent.
It is rather weak as anyone can lie about their age. However, the alternative would be that we'd all have to prove our age. Age verification, I think, is more about making sure that TalkTalk is compliant with the GDPR and doesn't open itself to huge fines.