Hasn't this question been answered like 100 times already? Hiruzen WAS in his prime, but the appearanceis that in which you were at the time of death. So although he appears old, he is just as strong as he was in his younger days.
As for Madara, Madara appeared young because his body was ALTERED BY KABUTO. Kabuto altered Madara and gave him extra Senju DNA, which somehow made him appear youthful in appearance. Also, Nobody would really take him seriously if Kishi drew him as an old, white haired geezer with Rinnegan.