I never really thought about it. I like Gordon to be a fair bit older. By Year One, Batman's in his twenties and Gordon is a Lieutenant who has moved back from Chicago and a veteran detective career there ... prior to that I like the canons where he was the beat cop on duty the night of the Wayne Murders, which was something I never really thought about until Batman Begins and was probably the strongest thing that story put forward, and I even like the idea that "Solving the Wayne Murders" was something that launched Jim into becoming a Detective, the way it was the crux of "Gotham", even if he never actually is the one to solve that case and ends up moving to Chicago for whatever reason.
But anyway, if he's a beat cop when Bruce is 10 years old, he's probably at least 15, maybe 20 years older than Bruce, and I think that's the right range and remains that way. Plenty old enough to have a daughter Babs' age, which is just a smidge older than Dick Grayson, but young enough that James Jr. wasn't like "wow this old guy still having kids?" Of course aging James Jr. up to being a young man and turning him into a serial killer was probably one of the most cliched, dumbest decisions I can think of - I'm not talking about the story execution, I rather like Black Mirror, just that I then and now think the aging up was silly and dumb. (It does always beg the question of; where the heck is Jim Gordon's little boy during all these years of stories, but hey, Frank Miller, can't always know what wrinkles your story might throw into "canon")
Presently if Babs is in her 20s (probably mid) and Jim and Barbara had her in his younger days as a beat cop in that kind of normal blue collar range of "first wife, starting a family", and all that, that tracks with Bruce being in his mid to late 30s and then Jim being in his late fifties or even early sixties. He's supposed to be a veteran, old, still in good shape but he's also had a heart-attack, been shot twice, can't ever quite seem to give up smoking, quitting cigarettes but then smoking a pipe then going back to cigarettes, so not exactly "peak" shape, and like a Detective and top cop in Gotham City would have to be, just running on all the caffeine and nicotine in the world trying to deal with all the Pop Criminal and Dark Occult Serial Killer crap being thrown on his desk.