Likely a rhetorical question, but I’m inclined to say Silver. For a couple of concrete reasons - the general rocketry vibe primary among them. But mostly because ages are defined in retrospect largely by outside observers, and in the case of comics, how a character is used by the following generations of creators can go a long way toward retroactively cementing classification. J’Onn’s been used as a signifier/mascot of that era in a few major projects from DC, which seems to indicate that thinking exists on an editorial level as well.

I suppose the true answer is something I didn't really think of before--the Golden Age and the Silver Age overlapped!  Captain Comet premiered June, 1951, but the final issue of THE MARVEL FAMILY was January, 1954--I would put the entire Fawcett line in the Golden Age.