This question (a good question at that) reminds me of the often political questions some new members ask. "If the world turned Objectivist tomorrow, wouldn't old people starve and children have no schools?" The world is not turning Objectivist tomorrow. Gradual change would be needed in the political arena. The same sort of change would have to occur in this question.
The intellectuals of a culture shape and direct its philosophy. In this way, those would have to be the first people to change. The effect would trickle down affecting every aspect of even a relatively unintelligent person's view of life. If this person grew up in an Objectivist oriented society they would have a very good chance of following the logic involved. After all, they would have been taught from an early age that their mind is important and how to use it. Some basic ethics could be understood by even a simpleton.
Out here in the current, real world, I have my doubts for their success. It is certainly possible, (we do have free will and all) but when one is surrounded with intrinsicism and subjectivism from most every angle since birth, it would take some serious force of will to come out of it. Building up knowledge from such a shaky foundation is tough. The best chance would likely come from Objectivist parents who sought out an atmosphere conducive to rationality for their child. I'm curious if there are any case studies of a sort that would test this?
Happiness is not the private reserve of the old, wise, or accomplished. They may have more fulfillment, but I would contend that you can be just as happy at 20 as at 60. It is not a choice to act happy as much as a choice to attain those values that you find important. As long as you are working toward having those values, you can enjoy life. Rand's novels often put the protagonists in epic struggles, only to come out victorious. We often live more murky lives and unless we are Galt we have to obtain the values available, sometimes only in increments. This should not diminish our real life accomplishments in any way. They are still good. It's not right to compare good to some sort of Platonic perfection.