Answer to Question 1
The higher the density, the younger is the universe. This is because Hubble's constant measures the expansion rate at present. If the density is high, then the expansion has slowed substantially with time and must have been much faster in the past. Faster expansion in the past means it took less time for the universe to reach its current size, so the universe would be younger.
Answer to Question 2
A faster object's mass increases and length increases.