Interesting question...

I believe, at age 22 that I differ greatly from who I was at age 18, for example.

Time passes, and although some changes in behaviours and interactions (and personality characteristics) are due to biological factors, some are also due to the way we're treated and the way others interact with us, and the responsibilities we're expected to fulfill (all as a result of our age, and people's impressions of what they can expect of a person that age, from either experience with others, or their own life experiences at that age in comparison).

I think it's mostly choices that impact how we change as human beings. I know of plenty of people who, at ages 30+ still act like children, whereas I've seen younger people act more mature than what is normally seen of individuals in their age range.

Some characteristics are age dependent though. We know, for example that many children at around 2 years old will act a certain way (hence the "Terrible Twos") and we know that teenagers are more apt to get into trouble and change from who they were as younger people, because they're experimenting to find out "who they are".

Some people will grow out of certain stages. Some people will be forced out of certain personality characteristics when confronted by others or situations. Some people will decide that certain characteristics they have need to be changed, and will do it willingly on their own. Still, some others might seek counselling...

And sometimes, some people will never ever grow up.