I am an 18 year old US citizen. I am planning on flying to Kazakhstan with a short layover in Moscow, Russia. I have read that I need a transit visa in order to fly through Russia, but I have also read that it is not necessary. I do not plan on leaving the airport at any time. Do I need a transit visa?
As long as the trip is in one booking, you don't need a visa.
Many sources claim you need it if in transit to Kazakhstan, with some claiming it's also needed if in transit to Armenia and Kyrgyzstan. In reality, though, it's only needed if in transit to Belarus, and the Timatic database that airlines' check-in staff use reflects this.