I think Johnny Depp has been suffering a spectacular decline, though this is multifactorial.
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You'd hardly believe that's the guy who was so beautiful in his youth.
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He's been balding for years, but seems to be keeping too much progression at bay (never >NW3) - probably meds and hair transplant.
One of the greatest looks declines of all time (for any reason) would have to be Edward Furlong. I was so jealous of his great hair and good looks back during the T2 era.
Then:
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Now?
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Still has great hair though. Thus proving that at least for what it's worth, there are plenty of different ways to become ugly as sh*t as you get old. Be creative! Mix and match! Fun for everyone!
As for hair loss, many people quote James Taylor who is still very handsome but went from Stud Dream Boat to just an old man:
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A more challenging question would be to find someone who actually became BETTER looking for any reason after their late teens or early 20s. Tom Cruise did it, but most don't. Most people just degrade with age in various ways. It's a downhill slope. That's nature.