Usually folks with the proper training and right make up of genes to succeed professionally will graduate fairly quickly through the ranks of USAC and end up in Cat1/2 after just a few racing seasons. One would need to start riding and racing with structured training starting at a pretty early age. Most pro's started in their pre-teens with a proper coach but there have been a few who started a little later.
Only a very small percentage of racers will find work racing, there are however many other jobs in cycling that could turn onto careers: sales, racing journalist, race photographer, team manager, coaching, race promotion, traveling and bike tours, sports massage, mechanic, fitter, owning a bike shop, etc. The list goes on. Not as glamorous as racing but more realistic in terms of landing.
If I were to guess, I would put the odds of becoming a pro racer in that same stratosphere as becoming a rock star.
As with any job, the power of networking cannot be under estimated. Motivation, perseverance, and patience will also be important tools.
Edit: This site may be a good start: http://www.bicycleindustryjobs.com/bicycle-index.asp