My sister-in-law tried to rent a bike at Disneyworld once, and was told she had to be at least 14.

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.