Weeks later, and we still don't need this.
If you want to build a city with age appropriate buildings (and GBs) for esthetic or other reasons, you can. No game changes required. You want a pretty city? Build one, I see them all the time. Again, no game changes required. Skins? For Pete's sake, do a search and you'll see why this idea gets shot down -- and ditto generally for ideas about age-appropriate cities, cities with no above-age GBs, changes to improve esthetics and so on. And weeks later, you come back with "educational opportunities" as a possible justification for making game changes to enable your Bronze Age - Iron Age game? Please. None of these things improve the game, they simply might satisfy the desire of the moment of the person suggesting them.
The Marble Mason already is age-appropriate, with the correct visual styling for its Age. That you keep it in Iron Age is your choice. If you want more production of marble, build more Masons if you must (vs trading down for it). But Inno is not going to re-skin and increase Mason production for you. They have much bigger fish to fry.