Question: Why didn't the lady who was originally with Mid-Size Sedan decay quicker, and we were able to see her body intact when she hit young Trent in the water?
Question: Why didn't Mid Size Sedan age sooner since he was there longer than the others?
Answer: I submitted this answer previously, but it was deleted for maybe appearing to be inappropriate. The reason for his slower aging is addressed in the film when he says "Black don't crack." Black people tend to age better than other people in real life, due to having a higher production of melanin and other factors.
Answer: Guests with medical conditions were given drugs in their drinks, and so they didn't age as fast as the others did.
The drugs had nothing to do with aging. The whole point was that they aged fast.
Question: How exactly did Kara get pregnant? She didn't have sex with any other character on the beach.
Question: Why didn't they need more than one pill? We take pills daily. These people took one dose the first day and then never again. They were there for about 50 years with one dosage but somehow they still worked?
Answer: The people were only there for about a day. The beach they were taken to rapidly aged them at the rate of one year every thirty minutes. The pharmaceutical company used them as test subjects by continually spiking their drinks with experimental medications for various health conditions to simulate a 50-year long drug trial within a 24-hour period.
Question: Why didn't the people going through the cave on the way in, experience the same mental effect as when they tried to leave?
Answer: It's just like the difference between swimming downstream and swimming upstream. It's tough to bring yourself out against the gradient.
Answer: Because she was 'freshly' dead when she hit Trent.