Rachel York was actually around 22 when she did City of Angels. Like many actors, she's fudged her age over the years. Still pretty young, of course, though.
Anthony Rapp made his Broadway debut at age 10 (in The Little Prince and the Aviator) and amassed a pretty solid list of theater credits while still relatively young, including Six Degrees of Separation and Precious Sons.
I believe Bobby Cannavale's son who is in Fish in the Dark is pretty young, too. 19 or 20?