Set cookie and redirect back to referral URL
Instead of relying on craft.request.getUrlReferrer() you could set a cookie containing the original URL before you redirect them to age verification page.

{% if getCookie('age') %}
    Page Content
{% else %}
    {{ setCookie('originalUrl', craft.request.url ) }}
    {% redirect "age" %}
{% endif %}


You can than use that cookie to redirect to the original link &amp; destroy it again once you verified their age. Destroying it might not be entirely necessary as there's not duration specified so the cookie only lives for the current session. 

{{ setCookie('age', '21', now | date_modify("+365 days").timestamp ) }}
{% set redirectUrl = getCookie('originalUrl') %}
{{ setCookie( 'originalUrl' ) }}
{% redirect redirectUrl %}