You're doing that thing I warned you not to do...
I go to bed every night not worrying about whether my company remains sustainable.
As do I. The key is to build the business to the point where it replaces your income before you quit your dev job. Once it's ticking away, there's very little that can happen over the course of the night (or a month) that can much affect its profitability.
The same amount of money appears in my account every other week whether I go on vacation, call in sick, etc.
Same here, with the addition of it still coming in if I decide to drop down to 20 minute weeks for the next year or so while focusing on something else.
You're right about the Ageism thing though. I do still hop back into the working world from time to time (to keep the skills fresh and buzzword-compliant), and companies seem more than happy to hire an experienced guy to build things for them.
But yeah, back to the point at hand: try not to scare yourself off by the "business stuff" of running a business. You can do as much or as little of it as you like, and so long as the product is good and your marketing is in place, it's just not a big deal.