There are lots of places where young writers can post their writing and receive constructive feedback. Here are a few!
This is my old stomping ground. This website has a strong focus on community and on constructive feedback. For those in New England, they also host events like workshops, readings, and poetry slams. My personal favorite writing organization (and not just because I’ve been working with them since I was 12).
Similar to YWP, but has always been national (YWP was Vermont specific for a while and only recently decided to go national).
My favorite storytelling website. An online art and storytelling project, this website is less focused on criticism and more on collaboration and sharing.
Every Day Fiction