If someone has martial arts experience there is nothing wrong with them sparring right away, but that is a judgement call the instructor makes. Throwing someone with no MA experience in with a senior student I find wreckless and foolish. Let someone take a class and than determine what they can do. I can watch someone go through one class and know if they have trained before or are blowing smoke. My two cents.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato