By a chain of searches.
https://en.wikipedia.org/wiki/Leif_Garrett But actually he's 56.
Ahem. Leif Garrett in Pico Rivera California on October 28, 2017 - Photo by Glenn Francis of www.PacificProDigital.com
Yes I know he's not the original singer of the song you discuss, nor the one in the photo, but...
Hmm. But I had a slightly younger sister, I guess. Of course, I'm conflating him with Shaun Cassidy, too.
Shaun Cassidy's wikipedia pic is not as honest.