Ok, I understand, but in theory we know he was born 35 years ago:
Not even something like "perceived" age like Bonney.
Also, even if it's not really super canon, he cut Z's arm off like 7 or 8 years ago.
I remember that Viver Card, but this is One Piece, Oda can come up with anything to explain why Weevil is a Kid living inside a 35 Year Old Body, especially with Vegapunk involved, it could be any kind of Scientific Experiment,
Momo & Bonney are just Examples, We also have those who Time Travelled, We have Brook who wasted so many years of his life doing nothing, We have Blackbeard who never slept which means his Mind is like that of a 60 Year Old, We have Clones or Vega's Satellites, We have Sugar & her Victims, We have Ope Ope Operation whatever it does, We have Gorosei who don't seem to Age, We have Zombies with Old Bodies but New Personalities ... etc
Oda clearly loves this Idea, so while i can't explain how Weevil's Case truly works, but i strongly believe He is just a Kid,
As for Zephyr's Case, Yes it could be Weevil who done that to him, but it's also possible that it's just Someone who Oda gonna say was Invited but Refused and will remain as just a Non-Canon Piece of Plot,
Anyway, i'm not saying you're wrong, nor am i saying i can't be wrong,
But i'm gonna keep my Expectations High for his Power (A DF as Powerful as Ace's or Crocodile's or Kuma's or Law's ... etc),
But Low Expectations for his Mental Age,