

I’m not aware of any study about veganism specifically, but in general (positive and negative) changes to your diet can lead to effects quite quickly, no matter the age. Eating better, drinking more water, being more active, to stop smoking etc. will all have notable effects on your blood levels in less than a month and - especially if you combine several measures - you will also feel these changes quite quickly.
Reagrding the topic of a healthier diet at a higher age, there are several studies out there. Just to name two:


















It clearly says…
I guess then A4 is supported as an input.