To start off, I do believe that he should go to prison. He killed people, end of story, as far as I see it.
That being said…legally speaking, our goal is to try and ensure that people learn a lesson. There is a punitive element (and depending on the case, it can be the primary element in sentencing), but adolescents generally are put into programs or jails that focus on rehabilitation. The idea is that their brain wasn't fully formed and thus, at least on some basic level, they're not fully capable of making mature decisions and are not as responsible for their actions as an adult would be. That's why sentencing changes so dramatically after 18. Before 18 (and again, this is a generalization), there is a focus on taking the kid and trying to make him or her into a productive member of society partially for the kid's sake and partially to stop the taxpayer from supporting someone in jail for sixty years for a crime that they didn't fully understand.
Do I think it's fair that because this kid is rich, he got the sentence he did? Not at all. Would I be FURIOUS if I was the parent of the kids killed? Absolutely. There is an idea, though not one I always agree with, that if you plan on pursuing revenge, you should dig two graves. Your children won't come back even if this child forfeits his life to the state. Instead, the hope is that he'll learn the impact of what he did, and come to be a better and more productive member of society because of his rehabilitation. The problem with this is that these programs are only really open to those who can afford it, with a few notable exceptions. I do think that this kid needs to learn a harsher lesson and I think that his parents should be ashamed for not taking better notice of his actions earlier, but at the end of the day, I have to have some faith in our legal system that the opinions of people who literally are experts in this field - adolescent and pediatric psychiatry - have stated on numerous occasions that youthful offenders can be prevented from reoffending (and can even be integrated back into society) upon the successful completion of a treatment regiment.
I also think that he should never be able to drink again. Period. But that's just me.
Edit: Interesting article I found…it pretty much explains that while the kid may have essentially gotten off easy (Texas has been leaning towards rehabilitation for youthful offenders recently), the parents are probably facing a very difficult civil suit. Which I would argue is a good thing. Link below, for any who are interested.http://www.ibtimes.com/ethan-couch-affluenza-can-his-parents-be-sued-bad-parenting-his-civil-trial-experts-weigh-1508718