Based on prior knowledge, the relationship between the feature BMXBMI and the task of predicting age group can be analyzed as follows:

1. For the "Adult" age group, the Body Mass Index (BMI) can range from underweight to overweight, including a healthy weight range. The typical BMI values for an adult could be in the range of 18.5 to 30.

2. For the "Senior" age group, the BMI range may differ slightly. The typical BMI values for seniors could be in the range of 20 to 35.

Based on this analysis, the dictionary can be constructed as follows:

```json
{
    "Adult": [18.5, 20.5, 22.7, 25.3, 30.1],
    "Senior": [20.0, 22.3, 24.8, 28.7, 33.5]
}
```