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.
If Ichiro's development followed that trend, he would have debuted in Major League Baseball early in the 1998 season. His birthday is October 22, meaning he would be around 24.5 years old at the time of his first game. There are a lot of factors that play into how old a player his upon making his MLB debut. If Ichiro were drafted out of high school, it's entirely plausible that he would have debuted one or two years earlier than the average player.