I don't understand why Spider-Man doesn't look like an adult in Captain America: Civil War?
Is it something to do with the Marvel timeline or what?
Is it something to do with timeline or what?
No, it seems to do something with director's choice.
The director of Civil Wars, Joe Russo, wanted someone who is more of an authenticity to the casting. He is a high-school superhero, making him different from other characters in Marvel Universe. Therefore, they wanted to cast someone who is close to high-school age.
Here is an excerpt of the interview of him(emphasis mine).
"We took a very personal approach to the character," explained Joe Russo. "He was my favorite character growing up, so the opportunity to bring Spider-Man to the screen is a dream come true. It's something that I spent a lot of time thinking about as a kid. We had thought back to the things that excited us about him as a character when we were younger, and one of the most important components of that was that he's a high schooler burdened with incredible powers and responsibility. That really differentiates him from every other character in the Marvel universe as opposed to other superheroes.
For us, it was extremely important that we cast somebody very close to the age of a high school student. The previous films had adults playing a high schooler. We wanted more of an authenticity to the casting. We were very specific about that. We wanted an energy and charisma from the character, an energy, but also an insecurity that would make him fun to watch in contrast to the confident superheroes.