When Does It Become A Mortal Sin To Refuse To Earn A Living?

Perhaps you're asking: "When Does It Become A Mortal Sin To Refuse To Do Servile Work?"

What if you willfully refuse to get a job - for more than a decade - and are able-bodied, able-minded and otherwise able, but you can't find a job that's 'worthy of you' and disdain service sector, etc. ...

...and choose instead to live at your parents expense - 100% - staying at their home, eating their food, paying for haircuts, gym membership, incidentals... with their credit card?*

I am referring to this objectively, not subjectively, i.e. not taking individual circuмstances, often unknowable by others, into account.

So, the notion of not finding work "worthy of your station" is interesting, because you actually see references to that in older moral theologians, where it does in fact factor in.  I would say it's no longer a consideration, but there were times in the past, with the class systems, that if you were, say, "nobility", taking on demeaning work would not be considered obligatory (though it might be praiseworthy to humble oneself).  Similarly, when reading about the obligations of husbands to provide for their family, there was typically a condition that they had to provide according to the woman's status/class/state in life.  I've never really agreed with that nonsense.  If some rich girl decides she loves some less-well-off man and gets married, provided he has enough means and works hard enough to meet the moral obligations required of raising a family, it was HER choice to "move down" in station, and he doesn't now have to work 4 jobs just to make sure she can still drive a Mercedes and shop for designer clothing, because her "noble family" was used to such.  It think that's nonsense, but I have read a fair amount of it in various theological thinkers of the past.

I didn't agree with it back then and don't agree with it now.

Now, based on your scenario, if the parents are OK with it, he's not committing any sin of injustice against the parents, and I'm talking if they're sincerely OK with it, not just (secretly grudgingly putting up with it), then there's no sin of injustice against the parents.  Perhaps they're rich and don't really care.  But, if it's hurting the parents, there's a sin there of injustice against the parents.  Perhaps they're enduring some hardship and aren't really thrilled with the arrangement but are just putting up with it.  So the injustice would depend on any impact to the parents.  Perhaps the parents want the child to stay with them for emotional reasons, so will happily accept the financial hit to have the company and support of a child (perhaps a mother especially).  Or perhaps the parents are getting up there in years and enjoy the reassurance of having a child present in case they suffer an urgent need, emergency, etc. that they can't do for themselves.

There could be some sin against sloth, again, depending on whether the individual does NOTHING.  There would be no sin if, while living with the parents who really don't care, he's doing something to help others, whether spiritually or physically or in some other way.  Perhaps the parents even relish the thought of helping to make that possible.  If I were a parent who was well off, I might tell a child to go ahead and volunteer to help the poor instead of getting a job, say, at McDonald's, and I'd feel as if I would be helping to make that possible by more-or-less financing it.  So the sin of sloth depends upon whether he's doing something (even if not for pay) or doing nothing but engaging in selfish pursuits, pleasure, relaxation, etc.

Finally, there could be a sin against any potential FUTURE family.  Unless one is morally certain that one will never need to care for a familiy, then one might be hurting one's future family by not saving money away from a job.  Again, obviously, if the parents are wealthy and would make sure the child and his future family are well taken care of (at least by dying and leaving him an inheritance, LOL) ... then that's not a consideration either, and we're back to sloth being the problem, and (if not engaged in helping people) sins of omission with regard to charity (both toward God and toward neighbor).

There are very few things in the realm of moral theology (unlike in dogmatic theology) that are absolutes that are not governed by prudence in their application, generally only the matters that considered instrinsically evil or inherently contray to Divine Law.  In this case, there are different aspects of things that must be factored in ...

potential sins of injustice or uncharity towards one's parents (if the parents suffer hardship or they object for other reasons)
potential sins of sloth (does not necessarily require a paid job to offset)
potential sins against future family (if not morally certain that he will never marry and if the parents aren't wealthy, for instance)

You'll notice that all of these entails an IF ... or UNDER [THIS] CIRcuмSTANCE stipulation, since it's not always an absolutely truisim depending on the circuмstances that prudence would then help navigate.