Despite suggestions that his age, 53, takes him out of the running, highly regarded Dennis D. Dammerman, newly named head of GE Capital Services Inc., could be the best-positioned insider to succeed legendary General Electric Co. Chairman and CEO John F. Welch Jr. when he retires in 2000. Dammerman, who's been senior vice president of finance and GE's CFO, has been a close Welch confidant and is now a vice chairman of GE's board and member of the company's executive office. What's more, although less temperamentally volatile than Welch, Dammerman is regarded as a first-rate strategic thinker. And if he succeeded Welch at age 55, he could have 10 years in the top GE job -- a period of time that's likely to become the new global norm for CEOs.