Don't join specifically and only for the retirement bennies, because that 20 year cliff vest is a steep cliff indeed. Being in the military chews people up physically, and spits them out broken. Get ill, or injured, and suddenly you're medically separated at 15 years.
Can you please elaborate on this more?
Sure. Not sure which part your specifically asking about, so this is a broad answer. Apologies if this covers stuff you already know.
1. The monthly pension element of military retirement is set up to incentivize staying for 20 years. There is no partial pension*. If you serve 17 years, and for some reason can no longer serve, you are totally shit out of luck.
2. If you are not longer physically and emotionally able to serve, you will be separated. If the medical problem is the direct result of your service, you'll get some disability pension. If the medical problem is not related to service, you will be medically separated with no pension. Sorry, buddy. Better luck next life.
3. Most service careers end up being pretty physical. You have to maintain standards, hump 50# packs, live under constant sleep deprivation, endure some pretty wicked stress, be held responsible for keeping people alive in hostile environments. Being an officer is a little better, being an infantry grunt is worse. The military has no incentive to treat you gently, because there is a constant stream of 18 year olds ready to replace you. As a 29 y.o. you're already past the physical prime the infantry is looking for. No matter your specialty, chances are extremely high you will end up hurt. The real gamble is on how chronically you'll end up injured.
4. The second way for you to get booted is the Up-Or-Out promotion system. Every few years, a cohort is looked at for promotion, based on your time in service and at rank. If you get looked at for promotion, and don't get selected twice in a row, then you can be involuntarily separated. In my service, only 50% of eligible contenders are selected for Commander. That means 50% of officers in my service get booted around 15 years of service, without any pension.
Take a look at
this article, about Military Pension being a helluva carrot, that only 17% of service members ever reach.
I know this post was quite negative. Again, I love serving my country. I don't have any plans to leave before that magic 20 years, but I'm realistic about my chances of never getting the brass ring.
*There are actually many exceptions to this, but none of them are under you control, and cannot really be planned for.
EDIT: spelling