(I love answering questions) When a magiphage dies, their body decays in a way that appears very similar to natural decay, except magical decay can often look like burning, as the magic can pop, crackle and glow from lack of mental control.
Magical creatures are full of magical instructions, spells, that build their bodies, as well as plain magical energy fuel and the actual stuff of their bodies. The body stuff and plain energy burns away or is eaten by fairies and other magiphages (they won’t hunt each other down out of courtesy for their own kind, but once it’s dead, food is food).
The spell stuff though, that dissipates into the surrounding magic atmosphere. It breaks apart in the process and mingles with all the other information that’s been left by other dead magiphages, and regularly an errant magic breeze will pick up these pieces and the pieces bond together, stacking one piece on top of another in a chain reaction. If the chain contains all the parts needed for a new critter with minimal omissions, the magic that started the process gets used to create them, and tada, a new magiphage is born.
The population is not always steady, often all the magiphages die and no new ones are born for a few years. Luckily the entire local population dying is not a big deal for continuation of the species, it’s like all the annual plants dying in the fall - doesn’t mean new ones aren’t going to spring up from the seeds left behind.
Magiphages life spans vary wildly. It depends on the strength of their spell build and how capable they were at surviving. Magiphages tend to slowly increase their lifespans in a given area over time. So areas prone to having them that are fairly new will see poorly adapted individuals living about five years, while old established hot spots and springs have individuals living up to a 100 years. The average in general is about 40 years.
The age of the place they came from matters because magiphage evolution is a tad lamarckian. They develop or lose traits in their lifetime in order to function better, and traits that limit their lifespans get lost over time as the spells are refined over multiple lifetimes.