I just watched it for the very first time, and I thought it was amazing, but had the trippiest ending ever...
From what I could make from it, it seemed that the monoliths were evolutionary devices helping humanity, the first one located on earth evolved humanity from apes into sentient beings, the second on the moon buried, waiting for humanity to develop to the point where they could recover it, and then when the second monolith was recovered it sent a signal to the final monolith, orbiting Jupiter leading humanity to their final stage, Dave enters the monolith and travels on a journey, modifying him, but first he has to finish his current form hence the aging, then when he dies, he is evolved Into humanity's final form, the space fetus.
That's what I made from it anyway, how about you guys.