Age is not indexed. And the birthdays are definitely 1 indexed: you denote your first birthday with 1, and not with 0, and so on. You don't denote any birthday

Age is not indexed. And the birthdays are definitely 1 indexed: you denote your first birthday with 1, and not with 0, and so on. You don't denote any birthdays of yours with 0. You may denote something else with it, but birthdays[0] gives an out of range exception. (Especially true in French, where birthdays are called anniversaire, but in English too.)