How do you propose we do that ethically?
Problem with self selection (identify kids who decided to watch porn in the past, and compare with those that didn't) probably will not be that huge, when the people are only outraged about "kids" self selecting to watch porn.
There are many researches done and still being done on how addictive drugs are, or on the negative (or positive) effects of drugs. Eventhough they are both illegal and often rather dangerous. You don't need to feed a healthy 15 year/old a daily near overdose of cocaïne to learn about the effects of cocaïne on a young persons' brains.
> I feel like in our society we're always beta testing everything before researching its effects, shouldn't we research the effects of internet pornography in kids before allowing them unlimited access to it?
Is that a rhethorical question? People of all ages capable of using it already have mostly unlimited access to the internet (and thus to porn). Sans parents understanding the internet more these days, and trying to set limits and failing.
Question should be whether somebody should do research before trying to limit access state/country wide in various ways. Ie. bigotry vs some informed decision making.