Showing Warning for viewing adult classified content
Do you want to implement the age verification for Anonymous users or for logged in users? If you are doing it only for the logged in users maybe a better option would be to just add a birthday field to the user and just verify the age on that, so you won't have to request the age all the time, or only the first time, and then store it in the birthday field.

If not, for the actual confirmation page, you could just use *hook_node_view* and if the user has not confirmed his age for that node, replace the content with the verification form. For the way you can store whether someone has confirmed his age for a node depends, as I've said above, on how you want things to work, for authenticated users, for anonymous users.