Before Finn and Jake met they were both adventurers. They both went to battle the Ice King and that is where they met.
Finn was 9 years old, and so was Jake. The ice king was 80 years of age. Before they all met, the ice king smoked every day and drank every night. [That's why he looks so old. Plus he is old.] Finn and Jake started talking, which they decided to be a team.
There will be other chapters later!