Structure Sorting in C / C++ with bool function

We have a structure and our element's Id and Age.

Now, we need to sort this Data,  

Rules:

If  Id is different, then students with higher Age get better rank.

OtherWise, get higher age.

Example:

Input Elements in our structure:

Id      Age

3       5

2       6

5       7

5       3

Output Elements in our structure:

Id      Age

2       6

3       5

5       3

5       7