It's never too late to do anything, unless you are six feet under! However there can be no guarantee where this career will take you 5-6 years from now. The industry is great at the moment for anyone joining with starting salaries over $50,000 year for the first year. With upgrades ranging from a few months to a few years, you can make close to $100K or beyond after a few years provided you upgrade. Movement to the majors would be dependent on where you are (flow, interview or off the street hiring).
Ask yourself how committed you are to the profession. The fact that you haven't flown anything in 18 years-- Was it financial, medical?
Get a medical, get hit the books, get current and then see if you still feel you want to give it a try. Doing it part-time will take up a lot of time to meet airline requirements. If you take too long, the climate may change when you get ready to launch. The time to do it is now and quickly. Even if you start at 45 plus, you still have a 20 year career assuming all goes well operationally and medically.