50 years if you go back to the beginning - but most people didn't get on the internet until the early 1990s, when it was "opened up" to commercial interests (which led to the meme of "Al Gore invented the internet").
Prior to that, the only way to get access to the internet (D/ARPANET) was (generally) thru educational or military connections; one of the few exceptions to this (I don't understand exactly how it worked) was being a member of The Well, which allowed some kind of internet access (not sure if it was limited to email, or if other access was allowed?).
The number of people on the internet prior to (roughly) 1992 were mostly confined to the United States, and was a relatively small percentage of the total population of the country, simply because of the exclusivity of the connections; it was open to commercial traffic. Prior to 1992, most people who had experienced some from of "online connection" did so via either multiuser private commercial networks (two of the largest in the USA were Tymnet and Telenet), or large "walled garden" shared systems like AOL. Then of course there were the myriad local and long-distance Bulletin Board Systems (BBS) - which toward the "end" (I put that in quotes because they never really fully "died") in the early-1990s started to standardize on form of "email" called FidoNet (IIRC, even the original developer thought of it as a hot mess of code that barely worked).
All of this is to say that for most people (but not all - certainly more than a few people here on HN likely had access to the internet prior to 1992) the internet is only about 30 years old.
Though you remain correct in the assessment that "we are no longer young"...sigh.