Hey Zebracake. I'm sorry to hear you're struggling with depression and physical pain, I can relate at least to the former as a motivation to try psychedelics. To answer your initial question: magic mushrooms will not directly affect you neurologically, though there is some evidence to suggest marijuana will at this stage of your brain's development. So you might actually be safer with psilocybin than with cannabis, in terms of physical effects. However, mushrooms are much more powerful than weed, so there's a guarantee that yes, it WILL affect you psychologically. Will it change your entire personality? This is very unlikely. But you can expect to have a different outlook on things on the other side of each trip.
I've advised 15-year-olds against using psychedelics so I can hardly condone it for a 13-year-old, but it's always going to depend on the person. I always used to consider myself intelligent and mature, though this self-image has changed of late, partly due to psychedelics... but I'm glad I started at 20, and not a year sooner. 'Tripping' is a very intense and confronting process and I'm sure it will have a far greater effect on younger minds still developing their preliminary worldview and sense of self than it will on adults. Please don't take this as condesencion, nothing pissed me off more when I was 13, but once you take a psychedelic drug, you'll understand the enormous gulf between a cruisy drug like cannabis, and an incredible six-hour mindfuck like psilocybin.
There are places where they give peyote and ayahuasca to 6-year-olds for shamanic training, and maybe that works for them, but that's a very different environment and they're given a lot of guidance and training for it. For a 13-year-old westerner, I can only advise that you wait, not for neurological but for psychological reasons. If you're determined to go ahead, the best course would be to start with no more than a half gram, likely to cause the equivalent effects of being mildly stoned, and work your way up very gradually from there.
A bad trip can scar you for life or drive you permanently insane. That's what you're dealing with here. Usually they result from taking too much in the wrong place and with too little experience. I've had two, and I'm lucky on both counts that I didn't completely fuck up my life, or lose my mind for good. One of those was on a mere two grams of mushrooms.
For your first experiences, a porch and TV with your friends might be okay, but shrooms are best experienced in daylight in the glory of nature. A sober friend and a long bushwalk would be your best bet. Choose wisely, hope it all goes well.
I remember i had a friend i met on this fps we were playing and he told me all about psychedelics. He told me he wiished he diden't mess with LSD at such a young age(15) because it clowded his judgement very much so and looking back he wished he enjoyed his child hood a lot more.
Zebra doesn't sound like he's enjoying what's left of his childhood all that much, if 13 can indeed be considered childhood. Devil's advocate, but I know someone with a heavily traumatic past who started with shrooms and LSD at 14; she hasn't regretted it. Different story for everyone.