Based on prior knowledge, the blood insulin levels (LBXIN) tend to increase with age. Therefore, it is reasonable to assume that higher LBXIN values are more likely to be associated with the senior age group.

Here's the dictionary with the specific details for the relationship between LBXIN and the target age group:

```json
{
	"Adult": [5.2, 6.1, 8.4, 10.3, 12.7],
	"Senior": [9.6, 11.2, 13.8, 15.5, 18.1]
}
```

Note: These values are only hypothetical examples and might not accurately represent the actual relationship between LBXIN and age groups. It is recommended to conduct a thorough analysis on a dataset to determine the appropriate LBXIN ranges for different age groups.