The actress rushed to his side when he was hospitalized last month.
"This will be a daily reminder for me to share my gratitude with those I love," Katie Cassidy wrote.
"Bye bye childhood."
Cassidy's role as a heartthrob in the hit TV show "The Partridge Family" launched his career.
There have been many other teen idols since David, of course, but none as big.
The former "Partridge Family" star is reportedly suffering from organ failure.
Dementia is not caused by one single disease.
The former "Partridge Family” star went public with his diagnosis days after a performance that left many concerned for his well-being.
The singer later revealed that he's living with early-onset dementia.
"Hey, hey, we're the Monkees!" This year marks the 50th anniversary of the phenomenon--you know, that show of an imaginary band that led to an actual band.