Once Dave has been through the Stargate and has made his way into the aliens zoo like containment suite for him he begins to age. The reason I call where he is a zoo is because there are subtle hints that suggest this like the noises in the background from strange creatures and his location in general. The room Dave is in looks like a mix of Victorian furniture but with a very sterile modern look to it as well. This is because the aliens have designed the room to look familiar to Dave so he feels as comfortable as possible. This is the same thing we do with the animals in zoos all around the world. It was a genius decision on Kubricks part to never actually show the aliens form because, for all we know, they are so advanced that they probably dont have any resemblance to humans at all. By not showing the aliens Kubrick allows the monolith to be our gateway to them when it appears to Dave on his death bed in his suite. The whole time Dave has been here hes been aging and learning. Right before he dies he points to the monolith is the same fashion that Adam is pointing to God in da Vincis The Creation of Adam Sistine Chapel painting. This is done on purpose to make the connection that, in the world of 2001, there is no God and the aliens are the highest form of being. This is where most viewers, myself included, really lost the film on the first viewing. After Dave points to the monolith he is transformed into the Starchild which is a glowing, fetus like being but also has the appearance of a man in the face. After this scene Dave is sent back to Earth by the monolith and the film ends as Dave, now the Starchild," looks on at our planet. The reason the Starchild was sent back to Earth is because the monolith has completed its duty of advancing human life to its evolutionary limits so it sent the Starchild back to continue its work. In the last 30 minutes of the film Dave is given all the knowledge of the universe and has been transformed into an immortal, all knowing being. Dave is now the films version of God sent back to advance his race further than they ever could have by themselves. Thanks so much for reading. I'm always trying to broaden my horizons with 2001 so I'd really love to hear your thoughts in the comments.