Hmm, I don't understand this...

So complicated

This code is not working with age >= 10, isn't it?

whoops, changed it

ah ok, understood. Do you get more honor from passing the full suite first try? Or is it just for stats lol.

Other than that, I'd argue best user experience is given when as much as possible feedback is available in sample tests.

Typo in description

perimeter(0.5, sqrt(2) = 2 + sqrt(2)

Should be

perimeter(0.5, sqrt(2)) = 2 + sqrt(2)   

Fixed

In the random test function

index = random.randint(0, len(text))

The end should be len(text)-1 so that the index doesn't go out of bound.

Finally added now

Oops, seems like I forgot to save them :)

No random tests.

I also added cursor positions to the examples, so I think the description is enough concise now

I added some more info about which word should be replaced and about word delimeters.
Is it enough?

There is specification, where I specified what does 'word' mean