The former are more or less set. The latter change upon time. This means that usually experience is more relevant when we're looking at skills than at attributes. The way abilities change is through experience. A given level of experience tells us how well a character is able to perform in a given field.
Experience is acquired.
This means that characters start with low levels of experience, and somehow are able to advance them. The absolute departing level in most abilities is close to 0 at the moment of birth. A newborn is almost unable to do anything. So, the character's level in each of his abilities is contingent on the moment in his life that we chose to look at. Usually, the more mature and the older he is, the better off he will be.