Board Thread:General Discussion/@comment-27599496-20200124065750/@comment-995426-20200124110037

Additional considerations: I don't know the exact answers to this, but at least there's plausible wiggle room.
 * It's possible that their mutation was later in the calendar year of 1970 than their first major mission together in the calendar year of 1984, which would be a a timespan of more than 13 but less than 14 years.
 * Issue #1 wasn't published as part of a regularly-scheduled, ongoing run, and the characters had already been in development for a couple of years or so. (There's earlier, dated concept art that can be seen in the pages of the First Comics reprints.)  So while it was published in 1984, it could have taken place in 1983.