"Thins them out" could be taken to mean he simply returns them to the normal world. Regardless of how you take that part, it doesn't follow in any obvious way that hook's pirates were lost boys slated to be killed who got saved by hook. In fact, since several seem older than Hook it's contraindicated.
It takes a lot of willful interpretation to get anything but killing out of "thins them out." We know what it means when somebody thins the herd, we know what happens when you thin out the crops, it never involves anything save killing.
Given that Neverland affects growing up weirdly, some pirates appearing older than Hook doesn't indicate much. They may have aged faster due to whatever thinking makes one grow up there, or could have been older before Peter got around to thinning them given Peter's sense of time is all kinds of messed up.
A kid who stayed a kid. But I'm going to hold out on the whole Pirates and Hook as former lost boys. Never heard that one at all.
It, I'll grant, requires a lot more reading between the lines than Peter killing ex-Lost Boys. However there's several things that point to it and it's a pretty common theory.
There's the odd connection that Hook and Mr. Darling are always played by the same person suggesting some connection there, but that's vague. Hook himself has odd qualities, such as being obsessed with Good Form but not actually knowing what Good Form is, which are suspicious. He also aspires to attend Eton college, a curious ambition for a pirate, which plays into his desire for good form (
He remembered that you have to prove you don’t know you have it [Good Form] before you are eligible for Pop.) Hook's last line is also Eton's motto. These qualities would make perfect sense for a Lost Boy who grew up and now wants to do adult things like attend college and isn't sure how, but don't make any sense if Hook is actually a golden age pirate.
The biggest one is Barrie's own line "All this has happened before, and it will happen again." Being as Hook and the pirates (save two) are all deceased at the end, this wouldn't be possible without somebody stepping up to become pirates and there's no more pirates in the world, hence where did the next generation of pirates come from so it could happen again?
And we do know it happens again due to the last chapter of
Peter and Wendy, after Wendy grows up, Tinkerbell dies and a new fairy replaces her, and Pan eventually shows up and takes Wendy's daughter and then her granddaughter, and things start over.
As you look at Wendy, you may see her hair becoming white, and her figure little again, for all this happened long ago. Jane is now a common grown-up, with a daughter called Margaret; and every spring cleaning time, except when he forgets, Peter comes for Margaret and takes her to the Neverland, where she tells him stories about himself, to which he listens eagerly. When Margaret grows up she will have a daughter, who is to be Peter’s mother in turn; and thus it will go on, so long as children are gay and innocent and heartless.