Welding for a living?

[Colorado Osprey]

I weld daily... but I am not a welder. We are always hiring people who can weld. I am very skeptical of anyone calling themselves a welder in a the fab industry.
I work in a fab and repair... steel, stainless and aluminum

Is there really a demand for welders?
Yes there is a demand for welders. Specifically in the oil shale and oil pipelines. I know people in these fields. Pay and demand is awesome. Conditions and hours are crappy. Most burn out within a year or two and go somewhere else.
Another great demand for welders is in the Gulf. Oil platforms and underwater welding. Platform pay is ok, again conditions are not. Underwater has some of the best pay and good conditions when not working, but is very hazardous.

For your questions directly:
What would a starting wage rage be? (I know there are many different types of welders out there, so this can vary big-time)
Non-certified welding pay could start at minimum wage. Certified welding of Titanium going into space pay is in the six figures.
Average pay for a welder is $17/hr or about $35k a year
Is some sort of certification required?
Yes and no. To weld you don't need to be certified. There is some confusion to certified welding because a certification is also for a job, not a person. Some employers want you have been certified before.
How long is welding school?
Most community colleges offer welding programs. They start with entry level in a semester and have more complex level courses. So, one semester or more.
To get steady work, do you almost "have to" be in a union?
No, good welders can get work.. they are sought after. I'm also NOT saying that only bad welders are in unions. Many a good welders is in a union.
Obviously, welding involves METAL, so are you busting your back all day? (I'm not exactly "young" anymore, but not too old to start something new...IMHO anyway)
Yes, I need two new shoulders and knees.... I have also had my ear drum burned out... and been flash blinded too many times to count.(late night burn)