at what point are we responsible for our susceptibilities?

I don't think we are. But we are responsible for how we cope with susceptibilities once we recognize them. When it comes to the cult, I don't believe many were at all responsible for the fact they fell for M's con.

If you want to look at hard questions here is one. Elizabeth Smart was about as innocent a victim of a cult as I can think of. Even if it was a small cult. She led the cult leader to her cousin's room and he nearly kidnapped a second victim but for a barking dog.  If the second girl had been kidnapped and raped was Elizabeth responsible even partially?

In her case I say no.

But an adult who joins a cult and then who like the Jonestown man who shot the congressman, murders at the behest of his cult. Does he bear some responsibility?

Ask Marianne? ( a joke ). I seriously don't think I have the wisdom for that. I do think that being under the influence of a cult is a mitigator though of at least some responsibility.

Some of the analogies you are using I don't believe work. I don't find addiction a very good analogy to cult membership.  I don't find sexual desires good analogies either.

At what point should one start taking responsibility for one's naivety, lack of wisdom and stupidity?

Never to the point of taking blame upon ourselves for when someone else victimizes us. We can learn from our experiences yes. We can learn that there are rapists in the world and we can learn to do all we can to protect ourselves. But that never turns into responsibility for being raped. 

Perhaps I have a problem with feeling anger where anger is due...

Or something like that yes, I think you may.  Anger is scary and blaming others for some people is harder than blaming ourselves. It isn't polite to blame other people, and it isn't polite to be angry. We are taught to be nice and self sufficient. Also, and I think more important, it is kind of like people who are victims of awful things that MUST find a reason. If we can find out why then we can regain a sense of control.

I think the point I'm trying to make is that unless we try and work out why we were so susceptible (OK, more susceptibility, less responsibility), we'll never really know ourselves, and unless we really know ourselves we're likely to go and make more messes  if we've got enough years left.  And that the reasons we were so susceptible, particularly if we stuck around for years, well into our 30's, 40's and even 50's, are well worth examining, particuarly as so many others abandoned ship in their 20's and 30's.  What took so many of us so long?

I would not worry so much about comparing yourself to others. I was 13 when I fell for it and 17 when I left. Pretty short time really. It had a huge impact though. But I really don't feel superior in some way to those who left later, or even to those who are still in. I guess I am proud I left on my own. I am not sure I am anything but luckier though.  Some of the well known reasons being fall for cults are really not character flaws. They just broke up with a girlfriend or boyfriend is supposed to be extremely common. The thing that made you vulnerable, or that made you snap out of it, may have very little to do with character at all. I am not sure that is a given. Maybe the people who got out earlier had experiences that randomly happened to help them snap out of it. I think you may be making some fallacious assumptions.

I need to go but I want to post this.