Hey, welcome to the forums! These are really excellent given your age. There are a few concepts that I think you are definitely ready to start thinking about:
1 - Priority/Composition - Evan brought this up already, but right now most of the elements of your images have the same level of visual priority. That is, when you look at the piece, it doesn't direct your eye anywhere - the foregrounds, characters, and backgrounds all have the same amount of color and contrast and detail. This can be confusing and unpleasant for the viewer! I think you're ready to start thinking about focus in your work - in the first image, one assumes that the motorcycles are the main subjects of the piece. It would follow that the bikes would have more contrast and more vibrant colors, compared to the fence and crane, for example. This is not an easy concept to grapple with, as it requires knowledge of color theory, comparative hues, bla de bla de bla. But it's very important, and definitely worth the trouble and practice.
2 - Care/Polish - There is a very sketchy feel to these pieces - jagged lines, rough shading, etc. They would benefit a lot from a second pass; the crux of pixel art is the idea that every pixel matters, and matters A LOT. Pixels need to convey as much information as possible, because they represent a lot of space in the image. For example, you have maybe a dozen or at most a couple dozen pixels to portray a motorcycle rider's entire hand! Each pixel needs to be placed with care, and you are ready to start thinking about subpixels and subpixel space. So imagine you are trying to pixel something very thin, like the chainlink fence right? In your piece, you have each pixel filled in completely black, because a pixel is as thin as it gets right? ...Not really
If you try making the fence dark blue, instead of black, you'll find that it looks thinner against the blue sky! Like #1 this isn't exactly a simple concept, it overlaps with anti-aliasing and some other things, but definitely something to start playing with. There are lots of good tutorials about stuff like this in the guides area of the general forums here.
3 - Like everyone else here, you'll benefit a lot from studying the subjects you're rendering, in this case people, environments, and motorcycles. Observation and recreation is the only way to really learn how things are actually shaped! More authentic representations of your subjects will be a good place to go from here.
Finally, as I said before, great start, and we look forward to seeing more work from you!
