Although the small yellow most likely was originally a size known as a peewee (usually less than 1/2 wide when made), the others show various stages of being worn down by grinding of rocks and pebbles.

The wear does not indicate how old a marble is. A marble that has been buried or protected from the waves can have very little wear and still be older than a worn marble.

As well as whole marbles, you can also find pieces of marbles like the two above.