I can't give you detailed advice without knowing more about how you've structured your data in tables and what relationships you already have. I can describe a general technique that might work.
There are a number of aggregate functions you can look up in the help file. These can calculate sums, standard deviation and count records much like your summary fields, but they use relationships to match a specific set of records instead of relying on a table's found set to determine which records to include in the aggregate calculation.
It may turn out that you can create a new table where each record stores keys that match specific subgroups of records as well as calculation fields that use the above aggregate functions to compute and display the values you need. Each record, in your case, might relate to records for a different age group.
I have not found out how to display teh data for both groups side by side
Filemaker is not very good at side-by-side. If you want to keep it simple, do your comparisons vertically, e.g.
Ethnic Group 1
- Age: 45 +/- 2.4
- Male: 56%
Ethnic Group 2
- Age: 44 +/- 3.1
- Male: 50%
This is very easy to do by creating a sub-summary part when sorted by group.