Wow thanks robhic...
One great thing about having been a JW is that you know you are not that smart... which in turn can make you a bit smarter.
To answer your question, I was about 13 when I became a JW. My parents had divorced a few years earlier, and due to the endless custody suits I had remained in a Catholic board school for several terms. It was a very sad period of my life.
When I learnt my father and stepmother (whom I eventually came to live with) had begun studying with JWs I didn't bite at once. However the first meeting in the KH did it. To me it looked friendly, nice, complex and critical enough (of other religions, especially Catholicism) to be true. And I wanted it to be true -- partly because it was a fresh family start. I instantly forgot all my doubts and objections (only to find them again later), read Make Sure of All Things in a few days, made up with my father and stepmother's Bible study, and was baptised together with them a few months later.
One year later or so it all went down: I loved school, both the studies and friends. I began to look at the religion with a critical eye, questioned things, turned inactive. But I was still deeply religious and felt divided. One night at the KH I broke down and thought I could not live without God. I prayed that Jehovah would "unify my heart" and started a desperate run from my own shadow. I left school to the despair of my teachers, became a pioneer, and ended in Bethel -- a place which really opened my mind (as many others here).
I guess the intellectual objections were always there, hidden (how many times did I dream of being on the other side of the door and raise true objections which no real householder ever came up with). In Bethel (especially in the translation department) we could express ourselves quite freely. But I did not take my own objections seriously until I somehow came to trust my own faith. Then the WT doctrinal stronghold went down suprisingly quickly.
I guess the true question is not how smart you are, but whether you are emotionally ready to trust yourself and pay the social price for it. Quite another issue.