They are what cheddar is before being formed into blocks and aged. Cheddar cheese is typically aged from 60 days to 4 years before being sold, whereas cheese curds are eaten straight away. While you can find mozzarella cheese curds, or other types of cheeses, it's almost always cheddar.
Players are signed and can be as young as 16. International players must be 16 to sign a contract. The universal age a player must be to play major league baseball is 16. The age limit is literally limitless. Once you get your high schooling it goes from there and you can be in the MLB which is usually around 18 for most people.
Each is listed showing his primary position; that is, the position or role in which the player made his greatest contribution to baseball according to the Hall of Fame. According to the current rules, players must have at least 10 years of major league experience to be eligible for induction. In addition, they must be retired for at least five years if living, or deceased for at least six months. Players meeting these qualifications must pass through a screening committee, and are then voted on by the Baseball Writers' Association of America (BBWAA).