Add another option in edited post


I need to modify 'edit post' form and 'new thread' form by add checkbox like "only 18+" (similar to "Disable auto wrap link")
I think I have to add column to xf_post table in database? Example in Install Add-One Class ?

Next when post is showing I have to check if its selected...and if its true then user who has set birthday in profile and who is younger that "18" then can't see this post...

Any advice ?