Couldn't it happen that the year was 0001?

Let's say we have a year 11100

That's the thing, this kata only tests 4 digits years.

The input will always be a 4 digit string

This solutions is slightly incorrect for it's not working for every given year.
Let's say we have a year (Just an assumption) 11100.
It will return a proper age but wrong suffix.
Your algorithm is not following a rule where every number with tens digit number has to be suffixed with 'th'.
Even tho, I think it's a very clever idea and it needs just a bit of polishing.

Btw. I know that my example is quite strange but I just think that algorithm written once should be able to handle cases for years without need of being rewritten. :)
I will optimize and correct my own answer and show it to the world.