I think it's unfortunate that some people have this unreasonable prejudice that the only valid Bible is the old King James version.  It just adds to the confusion.  In the NIV study Bible there is no reference to the "flower of her age".   That part of the passage is translated "if she is getting along in years"  The footnote explains that there was a lot of hostility toward Christians in Corinth at the time of that book's writing and some hesitated to marry at all.  There was also a possible alternate translation that  it may have been addressed to fathers who were reluctant to let their daughters marry, but should maybe reconsider if they were "getting along in years".   No, no magic age.  But young women probably were married by late teens and considered past their prime for marriage if they made it into their 20s still virgins at home.  That was true right here in our country up through the early 20th century