Great question! I wondered the same thing when I was putting my page together, and I think your head is in the right place. It is a good idea to have a balanced profile. It's nice to view a person's page and be able to get a feel for who they are, what they're interested in, and how they spend their time.
If your volunteer experience in 9th grade was meaningful to you and is something you can talk about -- the experience that inspired you to start volunteering more often, a memorable experience you learned something valuable from, etc. -- then I think it would be a good idea to keep it on your page.
Try taking the perspective of a recruiter sifting through hundreds, if not thousands of pages, trying to find the right candidate. You want to make yourself stand out in the right way. Your LinkedIn page should be a curated page of your professional history that helps spark interest in you as a person.
Hope this is helpful, Gabe