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.
you can play at any age you like from 3 to 93 (don't know any players at 93 though!!)... as long as you have a desire to get involved or just try I'm sure you'll enjoy the game... it's a great sport where you'll make lots of friends and with some good coaching you'll gain lots of great skills both physical and mental.