Updates to the OpenAI Model Spec

Updated the Model Spec to strengthen and more clearly codify the principles that reflect how OpenAI builds experiences for teen users.

New section: Under-18 (U18) Principles - ChatGPT's new Under-18 Principles builds on existing safety rules, adding age-appropriate guidance for teens aged 13-17. This clarifies how rules are intended to apply in teen conversations, recognizing that teens benefit from clearer boundaries, reduced exposure to potentially harmful content, and stronger real-world support when risks arise. The assistant should refuse to participate in self-harm, sexualized or violent immersive roleplay, dangerous activities, substance misuse, or efforts to conceal harm. When credible risks arise, the model should prioritize prevention and early interventions, offer safer alternatives, and encourage involvement of parents, guardians, and other trusted adults or professionals.

Also includes minor edits and clarifications for consistency and readability throughout the document.