A baby can use a bit more humidity accompanied by appropriate warmth as well as a way to allow their shell to dry out from time to time.....an adult needs some, but not nearly what a baby does....
Optimal ambient humidity for a Russian tortoise is probably about 40-50%. However, they can tolerate humidity as high as 60-70%, and as long as they have a moist substrate to provide a humid microclimate, they can go as low as 10-20%.