Minimalist Age Verification Overlay

I need a clean, minimalist overlay that asks visitors to confirm their age before they can interact with the rest of the page. The modal must appear as soon as the site loads, dim the background, and place a concise question with two buttons in the center. If a visitor indicates they are underage, the modal simply closes and the page remains exactly as it was—no redirects, no additional messages.

Please build the modal with lightweight, vanilla JavaScript, HTML, and CSS so that I can drop the code straight into an existing page without altering its current layout or performance. The overlay should be responsive, keyboard-accessible, and easy for me to restyle later through a single CSS file or block.

Deliverables:
• Complete HTML, CSS, and JS for the modal
• Clear integration notes (where to paste the code and any IDs/classes to add)
• A single line I can edit to change the required age should I need to update it later

Once I place your code on the page, visitors who confirm their age should continue browsing uninterrupted, while those who do not will simply see the modal fade away.