That is a good possible theory to have on the age speed.
Or you could also have it that the aging process slows after a certain age. Like say after thirty, when the body is beginning to start it's descent in age appearance. Wrinkles slowing showing, eventually gray hair ect ect. Because Miguel is twenty-three and obviously uses magic a lot but he clearly doesn't look half his age.
And upon the age where perhaps magic in your body, whether you use it often or not, naturally slows down the aging as some defense to help you live longer so the magic can live out a proper life span in the circle. Or magical puberty.
And then the Witches can have some kind of festival in honor of the change of the witches who will now grow slower than humans for its a big deal. Sort of like bar/bat mitzvahs or Quinceaneras, a coming of age of sorts for Witches.
Maybe?