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.
Article X. Section 1 (b) (i):The player (A) is or will be at least 19 years of age during the calendar year in which the Draft is held, and (B) with respect to a player who is not an international player (defined below), at least one (1) NBA Season has elapsed since the player's graduation from high school (or, if the player did not graduate from ...