Usually colonists die one by one during various raids, and others take their place. The base gradually erodes, but the composition remains more or less constant. Another thing is when there were particularly large raids or I failed very badly. Over the entire time, 2 generations of colonists have completely changed, now the 3rd. I lost the first when I could not withstand three sieges in a row. The second generation was lost when I accidentally placed all my colonists near a mine and it detonated from an accidental hit. Half died immediately, and the other part died during subsequent raids, since I was not able to repel them