Life and Death
In the original OS-tan canon, there were no deceased OS-tans, and little mention of death. The closest thing to this are a few strips set in the OS Wars depicting 95-tan nearly killing Mac-tan, referencing Apple's near-demise between 1996-98.
The first known mention or proposal of deceased characters was in the OS-tan Annex Project, the foundation of the OS-tan expanded universe. Among the OS-tan expanded universe fandom, it's accepted that in most cases, for an OS-tan to be deceased, they must not only be discontinued by their system's developer, but also largely forgotten by its former userbase.
OS-tans do not age like normal humans; they age very slowly, but their lifeforce fades over time after being a discontinued system. They are kept alive afterwards by their system's remaining userbase, emulation and original hardware. Occasionally, there are a few OS-tan deaths for storyline purposes.
It is the proprietary OS-tans that are the most vulnerable to death, since Open Source OS-tans represent systems that can still be freely modified even after being discontinued by their original developer.
Creation and birth
Very little is known about how OS-tans (and computer-tans in general( are created and/or born. A few theories have been mentioned, but none have ever been written about in-depth.
Computer-tans created by humans
One commonly-depicted way of OS-tan creation involves being made "from scratch" by human creators. This is how the earliest computer-tans were made, and even in more contemporary times a large number of computer-tans exist without having any sort of ancestral line.
Computer-tans born to computer-tan parents
OS-tans are often depicted as being able to reproduce asexually. Using advanced scientific or magical techniques, [source] code can be extracted from an OS-tan's body and used to create offspring. The extracted code from an OS-tan can be changed or enhanced, giving the offspring specific physical, mental and magical "mutations"; very rarely it is left alone, resulting in an exact duplicate or clone of the OS-tan in question.
While source code is usually obtained legally, sometimes the code is stolen and used to create unauthorized clones or children. This was the case in the creation of UNIX-tan, who was created from Multics' code without her knowledge or approval; SABRE-tan, who was created from SAGE's code; and in the Annex universe, Windows NT-tan, who was made from VMS and OS/2-tan's code.
OS-tans do not gestate their young. Indeed, an OS-tan doesn't even need to be present to have a child created, as evidenced by Unix, who managed to "mother" dozens of direct descendants with little personal intervention; and Linux, who has perhaps hundreds of children created from her openly-available source code. It is unknown exactly how OS-tan offspring are grown, but they seem to undergo some sort of accelerated maturation - the youngest OS-tans within the fanon and Annex universe(s) are depicted as being elementary school-age, with no known fanon depictions of baby or toddler OS-tans existing.
It has been speculated that any sufficiently compatible computer-tans of the same species (OS, hardware, program, etc) could produce a child together. However, there are no documented cases of computer-tans reproducing in such a way. The only fanon example of an OS-tan produced from two parents is NT-tan. The yet-unpersonified PDP-12-tan would be another case of a computer-tan derived from two parents - in her case, LINC and PDP-8-tan.
Difficulties of classifying computer-tan lineages
There are no set guidelines, even in fanon, regarding what separates direct ancestors (such as mothers and grandmothers) from elder siblings. As such, it is not uncommon to see OS- or computer-tans who personify ancestor/descendant systems portrayed as sisters rather than mother/daughter. In canon, the various Windows-tans provide a good example of this - in spite of the various Windows releases being sequential (for instance, Windows 2000 being the ancestor of XP, which in turn is ancestral to Vista), the Windows-tans are portrayed as sisters rather than a long line of ancestors and descendants.
Sometimes the term "modified clone" is used to denote the child of an OS-tan, particularly when the parent OS-tan has little intervention in the child's life, or is treated as a sibling to the "clone" rather than a parent. PDP-1 and her "modified clones" - PDP-7 and PDP-4 - are a good example of this practice.
Factors that determine remaining life force, decline, and death
An OS-tan is in her prime when she is a current system; growing in influence, power and code-wielding abilities as more hardware is available under her control. She also draws power from her direct descendants (younger sisters or daughters, etc.) she shares a code pool with.
Over time after an OS-tan's system is discontinued, the hardware she once had control over will fail over time, and her power will diminish, with eventual decline in sanity or physical abilities depending on the scope of her fall from power. Eventually she will die as she has no hardware to draw her power from, and can't draw enough power from her descendants to stay alive.
Not all abilities are lost though, if an OS-tan has natural abilities independent of code-wielding (i.e: 95-tan's swordsmanship, OS/2-tan's prehensile hair). Those natural abilities won't be lost, but an OS-tan with them may lose the strength to carry them out if she has suffered physical and/or mental decline.
In most cases, an OS-tan is considered dead if there's not enough of a userbase using just original, unemulated hardware and software to keep the system alive.
Exceptions
Some OS-tans are considered living, despite all their real-world systems being shut down and not being able to be run on modern hardware or in software emulators. Multics-tan would be the most notable example of this - her death was based on the real-life shutdown of the last Multics system, and her rebirth was sparked by Bull opening Multics' source code. Originally, Multics-tan wasn't going to be considered reborn until a bootable Multics system was created in an emulator, but the storyline implications of a reborn Multics-tan were too appealing to ignore. So Multics-tan is considered alive, although there is no good real-life basis for this choice.
On the hardware/software hybrid side, SAGE-tan is considered living, in spite of the last SAGE system being shut down close to 30 years ago and no full AN/FSQ-7 computers existing intact. SAGE-tan is alive mainly because of storyline purposes; however, there may be an in-universe basis for SAGE's continued life.
Emulation
OS-tans who personify OSes that no longer run on their original hardware, but are virtualized or emulated on modern computers, are usually considered alive. There are a few notable exceptions to this rule, however. (See storyline purpose deaths, below).
Storyline purpose deaths
Despite their real-life OS or computer still existing in some form or another (such as emulation or replication), sometimes an OS-tan is considered dead for storyline purposes. An example of this practice is Xenix-tan - initially, Xenix was considered a "dead" OS, so Xenix-tan was subsequently considered dead. It was later discovered that Xenix still existed, mostly in emulated form. Xenix-tan is still considered a deceased character, however, because her death played an important role in the backstory of the Unix family.
Necromancy
Necromancy, among OS-tans, is generally considered a cultural taboo. The reason for this is because it is generally perceived as a huge insult to descendants of the revived, as most forms of necromancy generally fails to restore most aspects of a -tan, and thus the "undead" -tan is often a very bleak shadow of her former self. However, notable exceptions exist from later days; in particular Colossus-tan, who is suspected to have been revived to power levels even beyond those she originally possessed - yet, at a great cost.
An important thing to remember when dealing with OS-tan necromancy, is that brushing with death sometimes changes an OS-tan's mind in quite horrifying ways; and while most who do lose their sanity do that as weak individuals on the very brink of their own destruction, a successful revival can often bring back their lives, without taking along their minds. With Colossus-tan, her innate coldbloodedness and killing instincts were amplified greatly, turning her from an efficient spy to a brutally effective assassin; in the case of WITCH, her curiosity, sense for problem-solving and teacherly strictness, turned her into a ruthless, sadistic inquisitor.