I think a lot depends on one's age and locale, since Creationism (specifically of the "creation science" variety) has a definite point of origin (the USA, because of the Constitution) and time of origin (again, because of the Constitution, as well as some other factors, such as the Jesus Freak movement).
The ACLU's attempt to get a US Supreme Court decision on banning evolution from the public schools for religious reasons failed when John Scopes' conviction for violating Tennessee's "monkey law" was overturned in the appellate court for purely technical reasons. That infamous trial had two effects on the anti-evolution movement:
1. It marked their victory over evolution, so now they could rest on their laurels and not need to be as publically active.
2. News coverage of the trial had exposed them to the general public as being the ignorant foolish rubes that they were, and that public shaming made them shy away from such exposure.
In addition, very shortly after the trial their dynamic leader, William Jennings Bryan, died, which both demoralized them and left them without a public spokesman, at least not one of anywhere close to equal stature. So for the next half-century they faded back into the background, exerting their pressure and influence at the grassroots level against textbook publishers and individual school boards and teachers, content that their "monkey laws" were still keeping evolution out of the public schools.
But then in the wake of the "science gap" signalled by Sputnik, evolution was reintroduced into the schools, which led to Epperson v. Arkansas (1968), which led to the striking down of the "monkey laws" as being unconstitutional. This got the anti-evolution movement moving again and, after their initial efforts were also barred for being religiously motivated, they invented "creation science", a deliberate deception to circumvent the courts by falsely claiming their opposition to evolution was "for purely scientific reasons and has absolutely no basis in religion." Throughout the 1970's, they ramped up their public visibility mainly through their travelling shows which incorporated both lectures and "debates". Then going into the 1980's, they expanded their ministry (their word for it, demonstrating just how non-religious they were) overseas to other countries.
And, of course, even before all that happened, they had been teaching creationism to their own for decades. But around 1970, burned-out hippies "got hooked on Jesus" and started flooding into fundamentalist churches, causing their membership to increase dramatically and turning formerly small churches eventually into mega-churches (Chuck Smith's Calvary Chapel being a local example) as they became highly virulent proselytizing vectors. This "Jesus Freak Movement" -- at first, "Jesus Freak" was a derogatory term used by outsiders, but then the Jesus Freaks embraced the term themselves -- not only provided an eager audience for creationists, but they in turn became vectors for spreading creationism as they proselytized the public unmercilously.
So then, how old you are and where you lived (plus whether you were raised in the fundamentalist community) would be strong determiners of when you first encountered creationism.
In my case, I grew up in an implicitly Protestant household, though I did not learn until I was in college that my father was virtually an atheist; he had become disenchanted by the rampant hypocrisy and as soon as he turned 21 he had nothing more to do with religion. I attended a Protestant church with our neighbors and was baptized into it around age 11. Then a year later I decided to get serious about it and started reading the Bible, though with a nave literalism that, thinking back, I don't think was part of our church. It didn't take me long to realize that I simply could not believe what I was reading, so I left. Further reading and learning some history showed me that I had made the right decision.
Now, growing up most churches seemed normal enough to me, except for the door-to-door types who were to be avoided, and my impression was that young-earth creationists were an extinct breed that had lived centuries ago. In high school, I remember one fundamentalist kid, "Brother Rumsfield", who wasn't popular. All I knew about him was that he had some strange beliefs; at that time (1968-1969), hardly anybody had ever heard the term, "fundamentalist", but that was about to change big-time when the Jesus Freak Movement exploded on the scene.
Family members of my friend became Jesus Freaks and it was through them that I was a fundamentalist "fellow traveller" and I learned more about their beliefs that I would care to know. It was in 1970 that I was first exposed to creationist claims. Besides the general claims of a young earth and Noah's Flood, two specific claims were of the living clam carbon-dated to be thousands of years old and of a NASA computer that discovered "Joshua's Lost Day." It was that NASA computer claim that sealed the deal for me. Even in those days of near-universal computer illiteracy I knew for a fact that no computer could have performed the magic that that claim required. I knew for a fact that those creationist claims were false.
I didn't encounter creationism again until 1981, when their travelling show came to the local university. I had to miss the show since I had duty that night, but it got me thinking. I was surprised that they were still around, which suggested to me that there just might be something to their claims. So I looked into it and the more I looked the more obvious it became that everything they were saying was false. It was also by that time that their political agenda against science education had become very apparent.
Since that time, I continued to study "creation science" and started discussing it on-line on CompuServe circa 1987.
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