Age is irrelevant with regards to effects on physiology, methinks. I thought I was lean at twenty, but then I saw some seriously lean folks and I knew I wasn't lean. I was padded. Not fat, just apparently lean but still padded. At twenty, I already had twenty years of growing fatter. At forty, that's another twenty on top. Then I went low-carb.

Had I gone low-carb at twenty, the effect would have been almost imperceptible, but still there would have been an effect. Also, losing fat mass was only one of the effects, all other effects would also have occurred at twenty, but then at twenty I was blissfully ignorant of pretty much my entire existence, so I probably wouldn't have noticed.

Experience makes one acutely aware of any effect, however small or marginal. So, age is relevant with regards to perception of physiological effects.

That twenty something guy you know? Give him another twenty, let's see what he says then.

On the other hand, if there is no potential for improvement, there will be no improvement, or there will be a detriment. However, I believe low-carb is always better than pretty much any other diet, at any age, for anybody, in any circumstance, for any purpose. Rather, I see no reason why sugar would somehow become really healthful at, say, 5 years old, and then become really bad for ya at, say, 40 years old, hm?