Originally Posted by
bat39
As you've rightly said, from a real-world perspective, the character of Superman organically evolved over time from 1938 to 1986. But at some point, around 1970(?), DC created an older doppelganger of Superman and said he's meant to be the character from the 30's and 40's comics, distinct from the 'current' version headlining the latest books. I think the Metaverse concept does a great job reflecting this paradigm - there is one Superman in the Metaverse who kept changing, but a 'backup' of the earliest iteration of Superman was preserved on another earth in the Multiverse i.e. the Earth 2 Superman. So this way, Kal-L is representative of the 'original' Superman, but the current Superman is also the original Superman who has evolved over the decades into his present form.
When it comes to the beginning of the Silver Age, Geoff Johns' stance is pretty clear - 1956. The events of Showcase # 4 to be specific. Barry Allen being struck by lightning was the beginning of the Silver Age from a real-world perspective, and in-universe too, it marked the first shift in the Metaverse, and the transition from the Golden Age continuity to the Silver Age continuity. He establishes the Silver Age Superman as debuting in 1956, having been born in 1931. He also depicts Silver Age Clark losing his father in 1949, around the same time as Superman # 53 was published (the first major retelling of Superman's origin, and the one which established the template for the origin that was broadly followed right up till Byrne's reboot). Superboy isn't mentioned by Johns, but it's not impossible to imagine he could have been active during the 1940's (which lines up with More Fun Comics #101 being published in early 1945).
It isn't just Superman - DC recently published a Silver Age Batman Omnibus that starts with 1956 stories. So clearly they've decided that 1956 is the cut-off point between the Golden Age and Silver Age. For Batman too, it kind of lines up pretty well - the Golden Age/Earth 2 Batman and Selina Kyle got married in 1955 and that's the first big point of divergence between the two versions. It's fitting therefore that the Silver Age Batman is now considered to begin in 1956.