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Show me that smile again!

Alan Thicke, who famously played Dr. Jason Seaver on "Growing Pains" from 1985-1992, turns 65-years-old today ... meaning, yes, he shares a birthday with both Justin Bieber and Ke$ha.

Since starring as the work-at-home psychiatrist, Thicke went on to appear on "Hope & Gloria," "iPod" and "The Bold and the Beautiful."

Alan, who is father to R&B singer Robin Thicke, still looks great -- but how has the rest of the cast held up?

Check out the gallery above to see Now & Then shots of Thicke, Tracey Gold, Kirk Cameron... and even Leonardo DiCaprio!

Also, check out the opening credits below, and don't be shy about singing along!

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