Odds of Hanging On
This very much depends on the kind of welding you're doing. A lot of people stick it out for the long haul. So many, in fact, that the national average age for welders is fifty-five (source).
Provided you're patient and careful, you can hone your skills and build a career. Growth as a welder depends on how many boxes you're able to check off on that big ol' list of welding techniques (source).
Even if you don't end up progressing, staying good at the thing you're currently doing will probably keep you employed for the foreseeable future.
The odds change when we return to the world of underwater welding. There aren't quite so many geriatrics when the job requires you to strap on a scuba mask and dive into the inky abyss, James Cameron style. In fact, most underwater welders are between the ages of eighteen and thirty-eight, and the majority quit the job after five to ten years (source). A solid fifteen percent die on the job (source).
Thriving (and surviving) as an underwater welder depends on care, experience, and attention to senior divers who know what they're doing.
Stay fit, focused, and not-on-fire.