To me, the key aspect of the Silver Age of Comics is that it was the only era in which comic books were seen as and intended to be juvenile entertainment. Certainly during the Golden Age, there were many comics that were aimed at older readers, and after the Silver Age it became quite common to explore more adult themes and concepts.
But during the Silver Age, comics were intentionally kept at a level where the stories would be acceptable to a mother of an eight-year-old boy. The violence was toned down, romance never got more hot and heavy than a chaste kiss, and real-world issues like race relations were seldom discussed.
Obviously the Comics Code Authority had a lot to do with that. But in the late 1960s the CCA began to be seen as a hindrance to comics moving to a more mature fare. Stan Lee famously dedicated Spiderman #96-98 (May-July 1971) to a series of stories involving drug abuse, and the CCA refused to issue their seal of approval. The comics went out any way, and were sold in stores. The CCA, exposed as a paper tiger, relented and a few months later gave their approval to a much more nuanced anti-drug story which featured the stunning cover of Speedy (Roy Harper) injecting himself with heroin.
Why the sudden push for more relevance, more violence and more (semi) nudity? When I have thought about this question in the past I just assumed that it was the famed Baby Boomers; that there were a lot of children born from about 1946 to 1956, that after that it declined fairly rapidly and regularly, and thus the comic companies were forced to chase the larger market.
Congratulations if you didn't buy it, because it's wrong. My first clue that it was wrong was when got live birth statistics for every year starting in 1952. I reasoned that probably the prime comic-buying years for a kid back then were from age 7 to age 12, a six-year span, so I looked at the number of kids who would have been that age in the following years:
As you can see, there is a bulge in the market around 1968, and a steady decline after that to 1972, but it's nothing dramatic, and 1972's market is still very close to 1964's, even if it's off about 4% from the peak. There's nothing in those figures to show why the comics companies were pressing to offer fare that would appeal to teenagers.
But this is one of those rare cases where the rolling average is hiding what was really going on. Here are the live births in the US by year from 1952-1964:
The story looks the same there; a peak in 1957, but still holding pretty steady as late as 1961, then some minor dropoffs in 1962, 1963 and 1964. Ah but look what came next:
Steep and sudden drops indeed; about 6.6% down in 1965, another 4% off in 1966 and and a 2.5% decline in 1967.
And the Birth Control Pill is the most likely cause. Although it was approved by the FDA as a contraceptive in 1960, it first became widely available in 1964-1965, following a series of court challenges to state bans on contraception. After the Supreme Court's ruling in Griswold v. Connecticut married women everywhere in the US were guaranteed access to contraception; the ruling was later extended to all women, married or unmarried.
Now let's go back to our look at the market. It's true that it held up fairly well until 1972, but then things really turned downwards:
So the companies were virtually guaranteed to be losing sales throughout the 1970s unless they could expand their market by appealing to a slightly older demographic. Hence the vastly different comic book world of the 1970s.
Incidentally, there is also a visible "Roe" effect as well when we look at the live births:
As you can see, births held fairly steady between 1968 and 1971, but then as abortion became more widely available there were swift and steep drops.
Note that I make no judgment here about the morality of birth control or abortion; I am just demonstrating that their effect on society was profound in ways that many of us may not have noticed at the time. And until someone convinces me otherwise, I'm going to say that the Birth Control Pill ended the Silver Age.