The interesting thing about Winter Soldier is that when Black Widow first teams up with Cap, SHIELD is not after her. She has no reason to suspect that SHIELD itself is up to no good. We can assume that Cap tells her about Fury saying SHIELD was compromised, about being jumped by SHIELD goons in the elevator, and the Quinjet. But you could say that at that point she's being flexible -- she knows Cap, enough to follow him on this chase out to New Jersey, but by doing so she's also keeping tabs on him. If it turns out that Cap really is bad, she can call in reinforcements. Her first inkling that things are not what they seem is when SHIELD fires the missile at the bunker. It's at that point that she goes in all the way with Cap, against SHIELD/HYDRA.
As for Spider-Man, Tom Holland is 19. They could have him playing 15 years old, but they could have him playing 18 just as easily. FWIW, both the Tobey Maguire and Andrew Garfield Spider-Men started their movies as age 18, in their senior year of high school.