They can take an active role in the prosecution of a case, but that means that it's usually either one that they have a personal reason to proscecute in court, or it's extremely important/newsworthy. Here's my personal experience: In 1993, my mother was murdered, and the case was handled in court by an Assistant DA. Later, when (for numerous reasons) a retrial was required, the same man was the DA; but he chose to take the lead position on the trial team; he felt a personal need to see it all the way through to the end.
A DA is unlikely to be younger than his mid-thirties. There are a couple of reasons for this. The first, very simply, is that it's an upper-level-management position. Those don't go to kids fresh out of school.
The second is simple math: to get a law degree first you start with a Bachelor's degree. That can take as little as three years, but four is more normal, and taking five years to your Bachelor's is becoming more and more common. (Means you graduate at around 22 or so), then a JD (three more years of school, minimum, longer if the person isn't able to attend classes full-time or needs to repeat any classes, so they're generally at least 25 or 26 when they get their JD).
Then they have to pass the Bar Exam; in most states, it's only offered twice a year, (for example, in Illinois, it's offered on "the last consecutive Tuesday and Wednesday in February and July". And you must
go to Chicago to take it.) And you have to file to take it well in advance (again, under Illinois rules, it's six months before you plan to take it.) This means that if our hypothetical lawyer graduated in May of 2010, the soonest he can take the Illinois Bar Exam is February of 2011 (He has to file the application by the end of July, 2010 to take it in February of 2011). Then he waits for the results; the February 2011 results are posted in early April of 2011. He's been out of school for nearly a full year — that makes his age 26 or 27, on average.
In Illinois, about 80% of people who take the Bar Exam pass on the first try. If he doesn't
pass on the first try, though, he'll have to wait another year, because he don't find out that he's failed until after the deadline for applying for the next time it's given.
He also has to provide proof of a passing score on the MPRE (Multistate Professional Responsibility Examination), an ethics test. That one is only offered three times a year, in March, August, and November. He can take the MPRE at any point while he's waiting to take the Bar exam, or even during his final six months or so of law school, so it probably won't cause any further delay.
So we've got a 27-year-old, newly-minted lawyer. Presumably, he's been working for a law firm that specialized in criminal law, or maybe he got really lucky and was working in an assistantship with the prosecutors office since he got out of school. So he's got some experience, and he's got the beginnings of professional relationships with the DA's staff. Now all he has to do is he has to convince the DA to hire him full time as a staff attorney, and rack up a few years more experience and get himself a good reputation. DA's are generally either appointed by the state's Attorney General or elected in an at-large election. Either way, he's not likely to get the DA position without at least
four or five years as a staff attorney/Assistant DA under his belt.
That's why you aren't going to find many DAs under about 35.
...if you don’t love you’re dead, and if you do, they’ll kill you for it.