Based on prior knowledge, we can assume that the age group for adults ranges from 18 to 64, and the age group for seniors starts from 65.

To analyze the relationship between the feature RIAGENDR and the age group, we can examine the distribution of genders within each age group.

Here is the dictionary with the specific details:

```json
{
	"Adult": [1.0, 1.0, 2.0, 1.0, 2.0], 
	"Senior": [2.0, 2.0, 1.0, 2.0, 2.0] 
}
```

In this example, the values 1.0 and 2.0 represent Male and Female respectively.