Why did ellen pompeo (meredith grey) age quickly??

I mean just look at her in the first episode, she was so hot, fresh. and then look at her in the third season, for example in "u don't get to call me a whore" speech, or meredith on morphine "what I am", she looked old, and it was like 3 years or sth, is thier job that stressful and hard???... show more I mean just look at her in the first episode, she was so hot, fresh. and then look at her in the third season, for example in "u don't get to call me a whore" speech, or meredith on morphine "what I am", she looked old, and it was like 3 years or sth, is thier job that stressful and hard??? or is it just genes or stuff ?!
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