Odds of Hanging On
Most defense engineers tend to stick it out for the long haul. The pay is good, the benefits are great, and the job can be incredibly rewarding. This situation is provided that an individual engineer can keep producing.
However, once an engineer stops being able to push the boundaries and either envision or develop the next generation of technology, then they'll be let go rather quickly. That stress of performance anxiety causes some to quit early. Others quit because of the politics involved or the results of their designs finally getting to them...but, for the most part, a defense engineer can expect a long career.