Based on prior knowledge, we can analyze the relationship between the feature BMXBMI and the target task of predicting the age group (senior or non-senior). 

Typically, a BMI value below 18.5 is considered underweight, between 18.5 and 24.9 is considered normal weight, between 25 and 29.9 is considered overweight, and 30 or above is considered obese. However, the definition of "senior" may vary in different contexts, so we will assume that a BMI value above a certain threshold indicates a senior age group.

Let's create the dictionary with the BMXBMI values for each target class:

```json
{
    "Adult": [20.5, 22.3, 24.8, 26.4, 28.1],
    "Senior": [29.7, 31.2, 32.8, 34.5, 36.1]
}
```

In this dictionary, we have included 5 typical BMXBMI values for each target class: "Adult" and "Senior". Please note that these values are example values and should be adjusted based on the specific context and dataset being analyzed.