Rosie, I don't know if you got the answer you were looking for, but I have done some checking on this subject. You won't believe what you won't find out. But here are some:
1) Elderly and Senior Citizen are usually used interchangeably. So any time you hear the term Senior Citizen you can think Elderly. I did find statements that suggest 65-70 years old are Seniors, however Senior discounts start as early as 50, some at 55, most at 60.
2) Both are generally a state of mind. Health, humor, activity, etc, determine this. (I personally don't like most 'appearance' statements. My husband and both my sons were completely bald in their early twenties. I don't think that means they were elderly. Also, I don't think people need to color their hair. How often have you looked at someone and said "Who do they think they are kidding"?
3) The only location I found that gave a definitive age grouping was the US Census. One category was "Older Population" which included people 55+. The next was "Elderly Population" which include people 65+.
So there is no answer.
As Phyllis Diller said: "Maybe it's true that life begins at 50, everything else starts to wear out, fall out or spread out."