I found an interesting problem on https://www.codewars.com/kata/5aff237c578a14752d0035ae:
My grandfather always predicted how old people would get, and right before he passed away he revealed his secret!

In honor of my grandfather's memory we will write a function using his formula!

* Take a list of ages when each of your great-grandparent died.  
* Multiply each number by itself.  
* Add them all together.  
* Take the square root of the result.  
* Divide by two.

## Example

```R
predict_age(65, 60, 75, 55, 60, 63, 64, 45) == 86
```
```python
predict_age(65, 60, 75, 55, 60, 63, 64, 45) == 86
```

Note: the result should be rounded down to the nearest integer.

Some random tests might fail due to a bug in the JavaScript implementation. Simply resubmit if that happens to you.

I tried it in Python, but could not do it. Can you solve it?
def predict_age(*age):
    return sum(a*a for a in age)**0.5//2