When are you able to change?
I believe anybody can change. Anytime.
The most common excuse - when someone cannot avoid admitting a personal flaw - is that personalities are immutable. "Aren't personalities set by age five?" they ask, laughing nervously.
It isn't difficult to think of people who have changed - and dramatically - well into adulthood. After marriage, the birth of a child, a divorce and other "life changes". Men become more emotionally expressive after retirement. And let's not forget therapy, or religious conversions.
All of these changes share something in common: There is a percieved pressure on the self. In most (save the next to last) it is at least partially external as well. This illustrates a truism I first heard from Peter Watts: A civilization develops technology in the face of adversity.
A civilization on a tropical island has no need of agriculture or goretex. Technology only develops - and takes hold - when it reduces antagonistic pressure.
As above, so below. Part of our "wiring" as animals is to search out comfort. We tend to not like disruptions - and especially those that cause us discomfort. So when things are comfortable, our psyches seem to hold to an "if it ain't broke" kind of philosophy. When things are broke, however...
..that's when things get interesting. That's when we develop new aspects to our personality. That's when we change to adjust to the environment around us. Our personalities, in a word, adapt when necessary.
Just like biological evolution, those traits that end up coming out were already there. They may have even showed themselves once or twice for a brief time. But there was no need to keep them, to keep that something new... and so the adaptation disappeared.
When the environment changes - when there is a percieved pressure upon the psyche - then those "recessive memes" come forward again so that we can reach a new equilibrium.
(To the one or two of you worrying that I sound like a functionalist... don't. Our journey isn't over yet.)