The concept of anonymity and the way threads are presented was meant to force the user to read ALL the posts and judge them by their own merit, not by some arbitrary preconceived set of standards, like appropriate terminology and prefered grammatical structure, not like this stops anons from doing so anyway. Remember when 4chan used emoticons and corrupt speech?
I haven't set up any filters but going but what I know about them, they'll always end up taking meaningful contributions just because they contained one word that you decided you didn't want to view--even if the very word was a secondary thing in the context of the post, like an example or a sarcastic remark-->Underage
The entire internet is full to the brim with underage, at no point in time was the average anon anything but underage.
If you want such a browsing experience I reckon a website with a strong vettinf process would be more suitable than an open anonymous imageboard.
Also,>Tying terminology to a specific age group on the internet