Together Puer et Senex or child and old man represent not only the earth-bound human condition of Adamic man in Christianity, but also quite specific planetary symbolism. The reason why Christian art absorbed pagan symbolism and why such syncreticism occurred during the Renaissance is discussed on the page devoted to the Layer monument.
The lower pair of Vanitas and Labor on the Layer monument can also be interpreted as highly-charged vessels of alchemical symbolism. In the realm of alchemical and astrological correspondences Saturn is the ruler of time and old age, melancholy, grey beards, agriculture and even of digging. A more fitting example of Saturnine attributes could not be found in the statuette of Labor with his highly expressive suffering features.
Similarly Mercurius, the messenger to the gods is often depicted as a youth or child, sometimes standing upon a rotundum in alchemical iconography to represent his world-wide influence and winged communication. The Layer monument's youthful Vanitas with his playful bubble-blowing could not be more allusive to Mercury in his symbolism. However most revealing of all as regards understanding the rich and complex symbolism embedded within the Layer mandala is the four-fold relationship between each statuette, human and divine, temporal and eternal.
If C.G. Jung had ever seen photographs of the Layer monument he would immediately have recognised the reason why Vanitas and Labor are paired together, for he noted-
Graybeard and boy belong together. The pair of them play a considerable role in alchemy as symbols of Mercurius. 
C.G. Jung amplifies the close relationship between Mercurius and Saturnus -
But the most important of all for an interpretation of Mercurius is his relation to Saturn. Mercurius senex is identical with Saturn, and to the earlier alchemists especially, it is not quicksilver, but the lead associated with Saturn, which usually represents the prima materia.... In Khunrath Mercurius is the "salt of Saturn," or Saturn is simply Mercurius..Like Mercurius, Saturn is hermaphroditic. Saturn is "an old man on a mountain, and in him the natures are bound with their complement [i.e., the four elements], and all this is in Saturn". The same is said of Mercurius. Saturn is the father and origin of Mercurius, therefore the latter is called "Saturn's child". Quicksilver comes "from the heart of Saturn or is Saturn......Like the planetary spirit of Mercurius, the spirit of Saturnus is "very suited to this work". 
 CW 9 i:39