Based on prior knowledge, we can analyze the relationship between the feature PAQ605 and the task of predicting the age group. 

Typically, seniors are less likely to engage in vigorous-intensity physical activities compared to adults. Therefore, we can assume that seniors will have a higher proportion of values of 2 in PAQ605, indicating that they do not engage in weekly moderate or vigorous-intensity physical activity.

On the other hand, adults are more likely to be involved in moderate or vigorous-intensity physical activities. Hence, they are expected to have a higher proportion of values of 1 in PAQ605, indicating their participation in weekly physical activity.

Based on this analysis, we can create the dictionary as follows:

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

The values 1.0 and 2.0 represent the possible values of the feature PAQ605 for the respective target classes 'Adult' and 'Senior'.