You can refer to actual photos of boys and girls in that age range and draw from them, observe and draw your own conclusion. Normally the younger the person is, the eyes and upper face are relatively larger. The more apart the eyes are indicate the age too. But if you're stylizing it, like all your characters' eyes are large, then I dont' have a simple way of explaining it. So my best idea is to look at actual references, and also look at your influences to see how they depict these age groups.

This class is so helpful, thank you! Almost nobody ever talks about collapsed or straight on gesture poses.

I think Proko talked about it in one of his classes (I think it's the gesture class in figure drawing) or the draftsmen podcast. Practice makes permanent. So if you draw lousily over and over again with wrong proportion, you'll just get really good at drawing lousily. Gesture is a slightly different thing though. It's difficult to nail the proportion in one minute, especially for beginners, and the point of gesture is not to have a precise drawing, but to get the spirit of the pose. I completely agree with your approach, and that's how I did mine too. Slow the time down, focus on expressing the figure with fewer strokes, rather than drawing really fast without thinking. And when the timer is up, use a different color pen/pencil to correct the proportion.

I'm not an anatomy expert, but have taken Proko's anatomy class. The front muscle over her strap is one of the deltoids. The back part that sticks out is probably the scapula itself, or muscles attached to the scapula.

You can always post and see if fellow learners can critique, especially if you have specific questions. There's no guarantee you'll get help, but could be a good way to document your journey.

Are you looking to animate rotating a cube? I'm not 100% confident, but when you first set your station point, where the observer's eyes are, you would have to decide where that point is. It's always on a vertical line down from the horizon line between the vanishing point. If you connect the station point to the vanishing point, there's an angle. A common one is 60 degrees. And then when you rotate the cube, the new horizon line starts tilting, and your new observation point would go with it, keeping the same relationship to the vanishing points. Hope that makes sense.

I think it looks lovely! What kind of final look are you going for? Do you have examples of artists that you admire? This can go many different directions. If you want it to look more vibrant like the photo, you can saturate some of the colors, like the tomato, onion skin and the lemon. Also, what's your focal point? Is it the onion or the cup?

Any pencil would sometimes have tiny crushed stones in it. Just draw back and forth on another piece of paper until that piece is removed.

Have you tried quitting and restarting your browser?

I don't have advice for you. I find it hard to remember too, but like you said, with time it gets easier. A lot of times I just feel/observe my own arm if I forgot something.