I personally (for better or worse) became familiar with Ayn Rand as a teenager, and I think Objectivism as a kind of extended Ayn Rand social circle and set of organizations has faced the charge of cultish-ness, and that dates back to, I want to say, the 70s and 80s at least. I know Rand wrote much earlier than that, but I think the social and organizational dynamics unfolded rather late in her career.
Albert Ellis wrote a book, "Is Objectivism a Religion" as far back as 1968. Murray Rothbard wrote "Mozart Was a Red", a play satirizing Rand's circle, in the early 60's. Ayn Rand was calling her own circle of friends, in "jest", "The Collective" in the 50's. The dynamics were there from almost the beginning.