Mr. Krabs keeps his first dollar on a wall in his house in "Wet Painters" (which is vitally important to the plot). However, in the episode "My Pretty Seahorse" (produced earlier but aired later), Krabs claims he spent his first dollar on a soda.
The show states that Mr. Krabs opened the Krusty Krab "after the war" (which war and when isn't stated; it could have been the Cold War, the Korean War, the Vietnam War or an underwater war which was separate from any human history), and he is shown to be an adult in the flashback. However, in Hooky, he reveals that the Krusty Krab has been around since at least 1959, which would have made him only seventeen years old at the time.