Why US Senators Are So Old
That's simple. As elected offices go, the Senate is #2 in the hierarchy, topped only by the Presidency. The House of Representatives is #3. A successful run for the Senate almost always requires that the candidate has held a long string of elected public offices. Very few Senators are elected for whom a Senate seat is the first or second public elected office they have held.
The most typical route for becoming a Senator is to start with a state legislative office or even a lower-grade elected local office. After building a reputation in a state house, the next step is typically a run for state senate, where you get to build even more political credibility. Then, you can run for a seat in the US House of Representatives. Each step usually requires multiple terms in the lower office to build that credibility to make you a viable candidate for the next level. After being in the House of Representative for a few terms, you can begin thinking about running for the Senate.
Each step up involves a larger group of voters who need to know who you are, what you stand for, and whether you are capable of doing the job you seek. In the case of a US Senatorial candidate, your constituency is an entire state.
The need to expand your political credibility means that you need to hold lower offices and advance through hard work that benefits your constituency.
So, that credibility building takes many years. By the time you run successfully for the Senate for the first time, you're likely to be in your 50s at a minimum. Once in the Senate, you can win re-election, primarily through valuable service to your constituency.
Being a US Senator is a job that demands a great deal of experience and public approval. That is true in almost every case. There are few exceptions to that path. John Glenn is an excellent example of someone who came to the Senate through a different path that wasn't political. But, there are few such examples.
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Since Senators have to run for re-election only every six years, voters get used to them being in that office. Incumbency is a very powerful thing in that office. That explains why we have so many superannuated Senators still in office.
Note: I wrote that last sentence just so I could use the word, "superannuated." 
"Few" out of 100 works. You're welcome to create a detailed list of those who don't match my statement. It will still be few, percentage-wise.
We had one from my own state, actually. For a while, anyhow. Al Franken certainly didn't come up through the ranks. He's not a Senator any longer, though.
Too many old people! We need a constitutional amendment! 
