Why is the age of 30 important in the Bible? Joseph, Ezekiel, and many more were all around 30 yrs old.
The Rashbam explains that at 30 one is worthy of leadership. L'Havdil in order to be President of the USA you have to be at least 35. Orach Chaim 581:1 mentions that a Baal Tefila for the High Holy days should be at least 30. The Mishna Berura explains that at 30 one is humble and broken hearted.
In addition to what Gershon wrote, the numbers 20, 30 and 40 can been seen as symbolic in more situations than just a person's age. At 20, one is just starting to be responsible. At 40, one has "completed" their transformation. 30 is the stage in between, where they haven't just started (gained wisdom and experience), and they haven't finished yet(they aren't perfect).