The current system on Twitch where you have Mature Audience setting, you really need to add a better Age Verification system than a button to confirm you are of age. To me this is lazy age verification, you need to have actual age verification before being allowed to watch streams that are marked for Mature Audience. It's such a simple process where you can confirm your age once with a third-party entity that can verify your actual age with official documents, so no documents are handled by Twitch directly, thus keeping the security and confidentiality of it's users sensitive information.

With proper Age Verification users don't have to be warned about mature audience streams if they've already confirmed the verification process. While also keeps those who aren't of age yet from being allowed to watch those streams. This would also protect minors on the platform, but still comply with regulations like the European GDPR on what data can be shared and collected.

As it clearly states in the ToS;

Twitch Services are not available to persons under the age of 13. If you are between the ages of 13 and the age of legal majority in your jurisdiction of residence, you may only use the Twitch Services under the supervision of a parent or legal guardian who agrees to be bound by these Terms of Service.

I think it's important to protect users who aren't of age, but have parent or legal guardian who agrees to the ToS on their behalf and allowed stream on the platform.