Though your setup will most likely work,
a more Kotlin-ish solution would be:

fun howOld(s: String): Int {
        return s[0].digitToInt()
    }

I'd also leave out the class for the solution setup and just take the function.

The test can also easily be written in a loop.

@Test
    internal fun testHowOld() {
        for (i in 1..9) {
            assertEquals(i, howOld("$i years old"))
        }
    }

Let me know what you think :)