Complex sentence: Costa and McCrae pointed out that these findings not only demonstrate good reliability of the domain scores, but also show stability (past the age of 30); scores measured six years apart vary only marginally more than the scores as measured a few months apart.
Split into simpler sentences: Costa and McCrae pointed out that these findings not only demonstrate good reliability of the domain scores, but also their stability (among individuals over the age of 30). Scores measured six years apart varied only marginally more than the scores as measured a few months apart.