faculty being able to spot outright plagiarism...usually the writing style is the dead giveaway

I can certainly imagine that the style of a 20-year-old student would differ markedly from that of a mature academic. But I wonder whether there would be obvious style-differences when the plagiarist is a mature adult? If not, plagiarism by an adult would be nowhere near as easy to spot.

I have no basis on which to answer that query; any thoughts?