Slug men do not live long... in theory. Their natural life-span is perhaps 50 years. However, the Tea Masters have long ago discovered the secrets of a life-extending tea. A slugman who drinks a cup of this elixir daily can expect to live 150-250 years. The slug-men call this simply "yellow tea" - their near dependence on it is a well kept secret.
There is now enough yellow tea for every slugmen... But it wasn't always so. Initially, the yellow tea was reserved for wizards. As supplies increased, the tea was given to other spellcasters as well. Today, the core of the tradition remains and is sacrosant: a slugman must demonstrate that they can wield magic (without items) in a short but critical ceremony called "the Proving" so that they can earn the right to yellow tea. Cheating invites exile.
Those who pass this important rite of passage gain access to a long life and the full privileges of being a slugman. Slugmen who fail to Prove themselves are second-class citizens, doomed to die early and aforded few chances to breed. Some become resigned and hedonistic petty officials. Others become driven, determine to find magic, or live life to the fullest. They often resort to crime, as they fear exile less than other slugmen - who fear losing access to the special tea. There is no time limit for a slugman to "Prove" themselves - if one manages to master magic later in life despite failing earlier, perhaps by finding a new (or ancient) form or magic, or by forging a pact with some petty god or entity, they will gain access to the yellow tea.
Some slugmen who haven't proven themselves take to eating large quantities of solid tea in hope to extending their lives and it somewhat works (gaining 10-15 years); this behavior is explained as "tea addiction".
Scholars have noted that the support of slugmen who fail to master arcane magic and who start worshiping various gods (and thus become holymen) plays an important part in maintaining the large number of shrine to obscure gods in the Yellow City - a single slugman's patronage can ensure a failing's cult survival.
The yellow tea does not work properly for humans. It halts aging for a decade or two, invariably followed by a fatal wasting disease. Some other alchemical means can prolonge human life for some times, but amongst archmage, these are insufficient.
The choice remaining are lichdom (wich few take), or undergo the ritual kown as Enftebtemang's Palmastic Transformation. This complex procedure results in the mage losing his head, and faces appearing on his palm. This may damage sanity somewhat, but does extend life by several centuries. Some say that when the two palms are joined together, the face changes, and Enftebtemang can speak through.
(See page 259 and 262 of the book for notes and illustration of such an archmage)
Design note: This was inspired by the notion that "all slugmen are magic users or holy men." Why are slugmen so eager to learn magic? Because it's a matter of life and death