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.
Joe Nuxhall makes MLB debut at 15. On June 10, 1944, 15-year-old Joe Nuxhall becomes the youngest person ever to play Major League Baseball when he pitches in a game for the Cincinnati Reds. Nuxhall threw two-thirds of the ninth inning in an 18-0 loss to the St. Louis Cardinals; he was pulled only after one wild pitch and allowing five runs on five walks and two hits.