"Can" expresses ability, permission, or possibility. Depending on the context in which the sentence is spoken, it can express any of these. Without any context, I would interpret it as ability or possibility, not permission.

Yes, "can" in this sentence expresses possibility. It means life has the potential to change quickly.

Yes, in the sentence "True! Life can change fast," "can" is expressing possibility. It means that it is possible for life to change quickly.

Yes, you've got the right idea. Life doesn't always change fast, but sometimes it does. There's a realistic chance that it might change. It can happen. It's a definite possibility.