What is it called when you can't tell a person's age?

Solution 1:

Illusive. I couldn't tell her age so her age was illusive, or her appearance, perhaps, was illusive, making it difficult to tell her age. I think illusive is the word but I didn't find it anywhere but from some obscure memory in my mind of having read it in a book once. :)

Solution 2:

While "indeterminate age" is accurate and correct English, it does not sound like something I would hear in conversation (USA).

Meaning exactly the same would be the more colloquial "uncertain age".

The most famous quote I can think of is Proust's "“She was "a woman of uncertain age.”

Note: "Uncertain age" is also used to refer to a period of time, as used by Robert Frost:

If this uncertain age in which we dwell Were really as dark as I hear sages tell, And I convinced that they were really sages, I should not curse myself with it to hell.

Robert Frost

"The Lesson for Today" (1942).

Also "Peter Pan Syndrome" which is not a standard phrase, but refers to the character in the story of Peter Pan who said, "I won't grow up" in the song in the musical.