Denny O'Neil's passing really got me thinking. It's at least arguable that his arrival at DC was one of the signs of/influences on the passage of The Silver Age into The Bronze. That was when Green Arrow turned from a Batman-Knockoff-ala-Robin Hood I to something distinctive. That was when Green Arrow/Lantern became Hard Travelling Heroes. That was when New Look Batman gave up his Adam West influences, embraced his roots, and started his climb from the brink of cancellation to his throne at the top of DC's ranks.
We could argue a couple of other dates as the SA/BA boundary, but the changes at DC in 1968 are serviceable sign post. You'd probably be hard pressed to argue a date more than 2 years earlier or 2 years later. Thing is, we can see the change from one to the other pretty clearly.
That change from Gold to Silver is a bit messier. Many would say the SA's signpost was Barry Allen's debut. Whether we accept that or not, when The Golden Age ended is even messier. Was it the change of All-Star Comics from superheroes to westerns? If so, what came in-between?