The Judaic and Torah view of this is vastly different. With regard to Torah specifically we acknowledge that it always arrives as a gift from heaven. One's age or mental acuity is irrelevant, Torah is open to all to master. There is no point short of death that prevents one from being able to master the Torah. The only prerequisite to enter into the gates of Torah is the will to connect to HaShem, the unrelenting will to keep diving deeper into the sea of Torah. This is so extreme that the Gemara makes a blanket statement that anyone who claims he tried and failed to master Torah should no be believed. If s/he really tried, they would have succeeded.
Rebbe Natan explains in Likkutei Halachoth (Hilchoth Tefillin 6:37-42) that while our body is impure, our intellect is incomplete, and until our body is worked like the leather of the tefillin, until there is no further repugnant smell, then our intellect cannot be trusted. Instead we must bind our will and emunah to the Tzaddik, whose body and intellect are refined so that he may guide us. For the spiritual intellect is something else entirely than the natural intellect which we relate to.
Since it would seem that real comprehension in Torah is dependent not on our natural intellect, but on our spiritual intellect, the only thing that limits our accomplishments in Torah is our devotion to the task.
In this way, we can understand how multi-tasking is virtually impossible with our natural intellect. Still, through deep study of the Torah we may begin to access our spiritual intellect which is not constricted and bound by the same worldly limitations that make multi-tasking a challenge. When we see things in this light, it starts to become clear why things that may seem impossible become entirely elementary when we connect ourselves to Chazal and rely on their testimony to the real possibility.
Shabbath is a perfect example. Someone who grew up in a Shomer Shabbath environment barely needs to think in order to avoid performing all 39 melachoth and their many many derivatives. Yet if you were to explain what keeping Shabbath entailed to an outsider, they might easily think that such a rigorous level of observance is impossible.
What I'm trying to get to is that performing even the most complex of the mitzwoth in the Torah has more to do with (1) accepting the yolk of heaven, (2) relying on and trusting in the Rabbis throughout the ages, and (3) faith in HaShem that He will bring about your success than it does any particular mental or physical aptitude.