Because posting a photo online is like getting a digital tattoo.
Once you post it, you can never delete it. It’s on there forever, traveling across the world on servers you will never have access to.
So, would you let your 12 year old get a tattoo? If the answer is “no,” then don’t let them get on Facebook and post photos. They don’t know that college admissions counselors are going to Google those photos. Future employers are going to background check them on Facebook and Twitter for years and years to come. And those are people who are actually looking with good intentions.
The entire world is going to see that photo, and though a small, small, small portion of that world represents their friends, a bigger proportion represents people who might not have the best intentions for that photo.