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.
Hopefully, the 91-year-old former Cubs outfielder will still be around to do it again when the Cubs return to the World Series. As of the end of this month, there will be 66 former major-league players who are at least 90 years old (Ralph Kiner will become the 66th when he turns 90 on October 27). Nine of those 66 played for the Cubs; some had very short careers, some had significant ones for the team, a couple played briefly for the Cubs but had more fame elsewhere (Kiner, for one, qualifies as that).