It implies that Dr. Pascal's inquiring mind and energy are youthful, but he is also experienced.
Explanation:
In this excerpt, the description of Dr. Pascal is a very positive one. We learn that the doctor is not particularly young. He is nearing sixty, and his hair and beard are white. However, we also learn that he is strong and vigorous, and that in his demeanour he resembled a much younger man. The phrase "a young man with powdered hair" wants to convey this idea. It implies that Dr. Pascal is very experienced, but that his mind and energy are youthful.
