I'm 30, am I in the age group with the greatest risk for diverticulitis and diverticulitis?

Not usually. The 40 and above is the most at risk population but 30 yo's can certainly get diverticulitis.
No. Diverticulosis is present in about 30% of people aged 60; this doubles by age 80. Only a small subset of people with diverticulosis develop diverticulitis. While we occasionally see people in their 20's or 30's with acute diverticulitis, it is overall very rare.