A homunculus is creature given birth through Alchemy by combining human genetic material with several special substances and making the resulting embryo develop without the use of a womb.
The concept of “age” is meaningless for them, as they receive from nature all necessary knowledge and reasoning at the moment of birth. The maturing of their bodies into adult stage does not take years like humans, nor do they grow any older from that point on.
A perfect homunculus is pretty much a man-made, artificial extension of nature that, external factors aside, will not die for as long the planet is healthy and has mana left. However, the price of being given life through such artificial means is that the homunculus will invariably have some sort of physical defect (small stature, short lifespan, lower intellect or lack of reproductive capabilities are just a few examples). Even with their potential longevity, they are below humans when it comes down to their strength as life-forms.
Despite having the same form and life as humans, homunculi are ultimately different types of existences, for they possess powerful Magic Circuits. Regardless how much of a fragile life-form they are, their Magic Circuits make it possible for the homunculus to achieve powers that are beyond humans. Fundamentally different than the magi that are humans with Magic Circuits, homunculi are better defined as “Magic Circuits with a human form”.
The homunculi created by the Einzbern, even those regarded as failures, have enough magical power to pose a threat to the Enforcers of the Association.