But in plants it's different, you can't take a general definition here. The body of animals and plants greatly differ. Animals have organs and so, they divide their body and the whole system must work together. Now plants just have shoots, roots and leaves, and they just recreate a copy of these in their search for either water or light. So they have this pronounced lateral or distal growth, and are able, in times of need, to destroy some of their older parts and then use some of the building blocks to grow more at the tops.
Oftentimes, "the fade" is attributed to senescence but in reality is due to a mismatch of a nutritional demand of the plant vs what's being fertilized. Outside plants that can survive the winter, like trees, they have to fade their leaves because the colder temperature make it gradually harder to grow. Even yellow leaves can still absorb light but when the chloroplasts have become dysfunctional then the chlorophyll-molecule will raise dangerous oxidative species, which can poison a plant from the inside out (it's the same as photodeath). Thus, these plants completely recycle their leaves given that the breaking down of the chloroplasts alone offers a great deal of energy to the plant.
Now annual species usually have a few timers in place that controlls (via the Circadian clock) that their lifecycle through spring, summer and autumn is optimized in order to carry seeds to the next year. This is done, for example, by an initial phase of not being able to flower until maturity is reached - this naturally prevents a young sprout to flower in spring (where days may also be as short as in autumn). Now for summer this danger isn't present (except for true aequatorial strains) and the length of flowering is set in such a way to still be successful before freezing cold arrives.
So this is where the typical working 6 weeks veg and 10 weeks of flower are derived for indica-type of strains or hybrids.
The aequatorial sativas usually are harder to flower out, have way longer flowering times and respond more sensitively to environmental changes such as the light regiment, spectrum and temperature, also fertilization. Usually growers can start them under short photoperiod already like 14/10, they may take longer to mature and may need 11/13 or 10/14 (longer nights) to incite them to not throw new flowers when the other parameters can't be controlled. These strains may even react to regional weather patterns like the monsoon.
Now nowadays you can have all kinds of genetics set everywhere, and most people don't know about the origin of what they grow. So all outcomes are very likely, like, as in the above example, the plant may have still expected to have another month before it would start to fade. The Circadian Rhythm is present in every plant cell and gives a timer of roughly 24h (+- a couple minutes) that itself can be reset daily by blue light. Apart from this impulse this timer is kind of very precise (for biological systems) and can even operate regardless of temperature or many other influences.
edit:
to further elaborate on this
and the length of flowering is set in such a way to still be successful before freezing cold arrives.
thing is, for plants it's not only important to just complete the seeds, but also to maximize the amount of seeds - as that then offers a greater evolutionairy viability. Alot of what plants do can be roughly understood by using "evolutionairy play theory", as the sheer amount of plants and their sessility forces them into this harsh optimization. So their natural incentive is grow and flower as long as possible, and then to finish at the right time.
And when that isn't possible, like far up north where there isn't much summer, then the autoflowers were more successful. So, since Cannabis was able to adapt to 3 totally different climatal regions on earth, the way it flowered also changed greatly. All this is just a reaction to the various environmental parameters at hand.
