The exact lifespan of these creatures is unknown, and probably immeasurable since none ever live long enough to die of age. Synx get themselves killed quite often, and given the dangerous nature of their preferred prey, it is amazing most synx live as long as they do. The only circumstances in which these creatures have an impressive lifespan usually relates to captivity of some kind. A synx can be killed just as easily as any person, and even easier in a few select ways, with the notable exception of decapitation. Their brain is located in what we could consider the chest cavity, (no vital anatomy resides inside the head). Like a cockroach, a beheaded synx is guaranteed to die of starvation, unless someone feeds it. When captured for scientific experimentation, it is often procedure to remove their heads as a safety precaution. On advanced worlds where synx are both prevalent and well known, they are slaughtered quite a bit, sometimes even for medical benefits. Many useful and unique things can be manufactured from their bodies, depending on the species doing it.
The gargantuan creatures that inhabit the synx sea-nebula often collide. Comparatively microscopic life forms that inhabit each of these creatures are self contained to the species they evolved on; excluding a few such as synx. Synx willingly move between these creatures when their enormous bodies touch, like ticks moving from person to person. This means that most animals synx encountered - even from the beginning - were biologically alien. Because of this synx evolved to be very adaptive to a plethora of biota: as both food and shelter. This however does not always work both ways, and on alien worlds a synx death can be quite unwelcome to an ecosystem, particularly the flora. Dead synx flesh is still technically edible, but the same could be said of aspirin. Most (not all) animals and insects see it as toxic and outright repugnant. In the most extreme cases the damage these bodies do is comparable to their weight in pharmaceutical waste; killing plant life in a small circle. When such a bizarre scene is witnessed by more simple species, it is often misinterpreted as supernatural. Synx can be almost as equally intolerant of flora, and will even regurgitate this substance if it resided in their previous meal. Vegetation is the only type of organic matter that does not exist in the nebula of synx origin, and their adaptability regarding it is extraordinarily low.
Their biology cannot tolerate too much sun exposure either; they die quite horribly and slowly if forced in direct contact with it for absurd periods of time. Even if this exposure is avoided entirely, a synx will still be exposed to all the other contaminants of an outside environment. This eventually leads to massive infections, many of which spread to vital systems and also result in slow death. The unusually strong and resilient become classified as weepers, however these individuals are very much atypical and present the smallest population of synx.
If such otherwise benign things as sunlight and plant life weren't comically awful enough, naturally decomposing flesh seems to have an adverse reaction to synx biology as well. Likely tied to the freshness they prefer in food, a rotting carcass is just as dangerous to a synx as plants or sunshine. The natural decomposition process of most animals on most worlds (such as earth) isn't like the decomposition process of animals in the synx nebula. In fact, it is so alien it's damaging for a synx to make extended skin contact with rotting flesh, something eerily hinted at with the fear and disgust synx show around corpses. Species aware of this vulnerability will use their own dead as weapons and barriers against these animals, especially if they are lacking in advanced weaponry. Pre-industrial worlds are especially fond of this tactic.
All Natural dangers aside, it should come as no surprise that the biggest cause of synx death is violence. Most civilized species who are aware of synx kill them on sight. Less developed races who don't know how to cope suffer more and have to live with this horrifying nuisance; losing children regularly in the dead of night. On most worlds where synx are prevalent, it's a common sight to see individuals surrounding and killing a synx to retrieve a body or exact revenge. Another big cause of death are the individuals who make it their profession to eradicate this menace. Often these groups are made of up adults who were attacked as children, or parents who lost their own. These are extremists effected by trauma, and they enact their profession with unparalleled prejudice.