I've yet to see an 18 year old that looks like an actual child regardless of how they're dressed however I'm also not looking for this sort of thing so...
This kind of thing happened a lot, and I think more than one entirely "mainstream" director in the 60s/70s filmed nude actresses below the age of 18. Fortunately it's a lot harder to get away with that these days. Not to mention (Weinstein and many, many others) sexually assaulting the actresses off screen.
Her entirely involuntary response was to exclaim, "Lady, I'm damn near 30!"
So yes; if a 28-year-old can be mistaken for younger than 14 without trying (or even wanting to), there are plenty of 18-year-olds who can appear much younger, especially if they are using makeup, acting tricks, and other means to artificially enhance that.