I love pearls and I think they can look quite youthful depending on the diameter of the pearl and the length of the strand. Anything 6.5 and smaller is young looking in my opinion and anything 7-7.5 is classic for both the young and mature set. I think a short strand of around 16" 7-7.5 looks very preppy and classic with a sweater or peeking out of a button down shirt.
I have numerous strands of different lengths and diameters and have been wearing pearls since h.s. although back then I wore a faux short strand and wished they were real. Since then I inherited a graduated akoya choker with a removable pave pearl drop (I wear on dressy occasions) and my mother gave me her akoya 7.5 16" strand that I wear more casually since it's a classic length.
I also have a long 6.5 akoya strand that can be doubled or knotted that I got for a steal on ebay, only $100 several years ago. That I also wear only for dressier times bec. it's long. Another one I got myself again on ebay is a fresh water 9mm size strand that is opera length and that is the only one that I feel is a more mature length and not very youthful both for the size of the pearls and the length (falls at bust line).
I would recommend a short strand somewhere in the 16" range, not bigger than 7.5 for a classic look that always looks fresh. I think it should fall somewhere around the collar or just below.