Why do highly educated people tend to look younger?

Looking at the people I know who graduated from a bachelors or PhD program, it seems the PhD students looked younger than the college drop outs or those who had BA’s but not advanced degrees and among my family friends, the more educated somebody was, the better they looked when they were older.

My theory is highly educated people tend to have better social relationships and positive social relationships makes you age slower.
Why do highly educated people tend to look younger?
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