We all know that there were young mutants in the Island facility at the end of "X-Men Origins: Wolverine".
More specifically:
"Gambit, Cyclops, and Emma were the only credited prisoners of Striker's Island. However, fans have speculated who the other prisoners might be: Quicksilver (white-haired boy who violently thrashes in his cage while bound by elastic straps), Banshee (teen with tape over his mouth), Toad (a boy with an amphibious tongue), Psylocke (an East Asian girl with blue and purple streaks in her hair), Whirlwind (young man spinning as a tornado in his cage, however, other fans believe this to be Riptide), Dust (South Asian girl seen frozen in her cage)."
I know that Origins wasn't filmed with First Class in mind, but let's pretend for a moment that the movies are perfectly lined-up and wired-up in the same continuity, okay? Let's imagine. Thanks.
Origins is set in 1983. "Apocalypse" indeed established Cyclops's age in an undeniable manner. He was 18 in 1983.
The spinning guy must be Whirlwind, because we already got Riptide operating in 1962 (First Class).
The "Psylocke" version must be the very same one we "later" meet in "The Last Stand" (set in 2000). She's Kwannon. Forget the LS end credits, she's not Psylocke, because we already got the real Psylocke in the altered timeline's 1983 (Apocalypse).
So my wild guess is that Emma Silverfox is Emma Frost's clone created by William Stryker Jr. in the sixties (DOFP established that Emma Frost was captured and killed) and later adopted by Mr. Silverfox, Kayla's father. Adoptive sibling.
The teen versions of Quicksilver and Banshee are clones too, of course. The real Banshee (First Class) was killed by Stryker in the sixties, while the real Quicksilver (DOFP) could have been killed by Team X down the line too, and that would explain why 2023 Wolverine knew about him and knew about his home address.
I don't think that they are "mutants with similar powers". Ironically speaking, and that's truly cool if you ask me, Stryker's secret files in X2 list two "Cassidys" and two "Maximoffs", without specifically mentioning their birthnames. Retroactive continuity, we can assume that the computer files referred to the clones.