What secret does Conway reveal to Mallinson? (chapter 11)

Conway reveals Lo-Tsen's age.

"Lo-Tsen is not young," said Conway.

Mallinson looked up and began to titter hysterically. "Oh, no, not young —not young at all, of course. She looks about seventeen, but I suppose you'll tell me she's really a well-preserved ninety."

"Mallinson, she came here in 1884."