I wondered this and it has been the case for my two boys.
DS1 was a very structured baby, liked routine, very into everything, interactive and wanted to be involved. As a toddler he was a damn hurricane, had no fear and went everywhere at lightning speed, and exhaustingly active.
As a 9 year old he gets stressed if his routine changes, asks a million questions, is very Ernest, needs someone to play with him a lot, still can't sit still, has to go everywhere fast, and gets the validation from human contact.
DS2 just sat and giggled for the first year of his life, watching the world go by. As a toddler he was easy going, cautious but happy to play by himself and very imaginative. He used to make up characters with his hands and play for hours.
As a 6 year old he's much the same. Happy to hang by himself and do his own thing, less active but has a wicked sense of humor and can understand the nuance of jokes that his brother can't quite get yet.
Both personalities are, at the moment, complimentary of each other thankfully, and they love each other fiercely!