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.
Amongst all teams in Major League Baseball, the Boston Red Sox have the roster with the highest average player age in 2014. The average age of the Boston Red Sox players in April 2014 was 31 years. The oldest player in the MLB as at April 17, 2014 is Raul Ibanez. The 41-year old plays for the Los Angeles Angels.