So first, I would say it would be best for the buttons on the main menu to be activated when clicking on them, rather than the mouse just touching them. It makes it more clear to the player what is happening. In addition, putting the buttons closer to the center of the screen would allow players to better see it. Making them bigger would also help.
When it comes to the tutorial screen, it would be much easier to use text instead of drawing the letters. You can either do this with the text tool in your art program and/or you can do it in the engine, depending on the situation. For godot, text nodes are called Labels or RichTextEdit. RichTextEdit Nodes are more complex and can do some cooler stuff, but Labels work just fine for most situations.
Your platforming controls are pretty solid. However, I do not understand how the grapple mechanic really works. If there is a cooldown to it, have a UI element that shows that cooldown. If it can only be used on land, then have a UI element that grays out when you are in the air, indicating that the grapple can't be used. In addition, the speed thing is very hard to understand. I think it's interesting to have a risk/reward element where going fast makes you avoid death, but make you slower when you get older. That is genuinely cool. However, when I dash, it just seems to drain my age and instantly kill me.
The grapple mechanic is cool, however, it conflicts with the "speed makes you die" gimmick of the game. Because it is very unclear how much grapple will kill you.
Here's what I would change about your game. Remove the grapple mechanic. It's cool but isn't needed when it comes to your theme of "speed makes you die". Double jump can stay, but don't make it kill you. Just make it weaker as you get older. When it comes to the dash button, instead of moving you forward, I think it should increase your speed, so you just walk faster. However, you have an age meter (or just two labels, one says age and one has a number) at the top of your screen. As you use the run button, your age will increase and you will slowly get older and your movement abilities will be slower. I think this could make things much more interesting. Another thing you could do is make multiple paths for level design. If your age is a low number, you can move faster and go through quick paths. But if you are slower, you aren't able to take as many shortcuts and faster routes. You could also have things which slow you down in the current moment like backwards conveyor belt or lasers. Making the reaper get closer to you and more likely to kill you. Also, having a life system would also make the reaper being an instant kill be much more threatening than it is right now with everything being an instant kill. But with obstacles that slow you, you need to make a decision if you want to walk through it but maybe risk the reaper getting closer to you, or running through it to get through that obstacle, but potentially harming your ability later on in the level.

Another thing is to not be afraid to do research on how Godot works. Some of the docs are not super great at explaining what things do, but I would not know what I know about Godot if I didn't research it. So be sure to look up how to do something if you aren't sure.
While this isn't a great game, I can see good ideas from this game which could be used to make this game better or a new game.