Yup, that one in the pic is a juvenile, and has a juvenile lip. Once the lip flares, the conch is reaching adulthood, and it will only thicken the shell and will stop adding whorls and length after that point. Most of the larger snails in this family (or at least the ones I've fooled with) reach adulthood in two or three years, and I believe the conch farmers claim three to five years for this species, although one source said two to three. Time to sexual maturity varies by water temperature with many snails, and the same may apply to Strombus gigas (I'm sure there's a conch farmer on this list that may correct, or verify the info?). It would probably be hard to grow enough macroalgae in an aquarium for these guys to reach optimum growth rates, but I suppose you might supplement their diet.
I've never tried to feed conchs, but maybe you do, David? Bill Gilly at Hopkins Marine Station kept a bunch of Strombus luhuanus for a while, and they ate macroalgae that was collected in Monterey Bay and dropped in the tank. You could actually see the snails tugging on it. Strombus gigas might behave similarly?
Don