The attention to detail to set the mood of the game feels like a cosy blanket and a warm cup of your favourite beverage. It is funny how seeing a character breathe with ease can be that relaxing.
Sound design is also on point. I read the message warning the player about the lack of music, but I personally didn't feel like it needed music. The birds, grass and wind were enough for me to enjoy the gameplay.
I experienced some confusion when interacting with the backpack and notebook in the UI. I'm on desktop so I tried to click on them. The controllers on how to open them appear when you pause the game, but since it is a slow-paced, no-danger gameplay you can easily leave it running without having to pause it (which cause that I saw the controllers only at the end of my play session).
I didn't get Trevor's age, but by the way they take notes (highly descriptive, image-based) and talks, I would say that not older than 9 years old? Which I think would be interesting to the parents who allow their young children play games during a long car ride: it is not over-stimulating and it does not have any "adult words".
I'll gladly wait for the full-game's release!