Solve the programming task below in a Python markdown code block.
# Task

Given the birthdates of two people, find the date when the younger one is exactly half the age of the other.

# Notes

* The dates are given in the format YYYY-MM-DD and are not sorted in any particular order
* Round **down** to the nearest day
* Return the result as a string, like the input dates
Also feel free to reuse/extend the following starter code:
```python
def half_life(person1, person2):
```