Yes they could add a bronze, steel and so on age or 'fantasy' ages, but I think because it hasn't bothered to go this route for so long it's likely why mods have stepped in and the 'fantasy/adventure/creative building' aspects of Minecraft have been added in past/future updates. Or the odd features like the recipe book/narrator/custom world gen changes.
Minecraft's update to me seem to add in more adventure or building aspects/smaller things that can be necessary (like in the case of iron nuggets) that still fits the name of the game. The new inventory blocks for 1.14 I find already somewhat fit the 'ages' aspect I think but differently where Villagers professions mean more and the new blocks add a next step to differ from the crafting table as it's own 'expansive tier'. Yes recipes will be new/the same but put towards a new block but the 'ages' effect seems to be hit here I think. So I think it's already getting it's 'ages' update already.
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Ages are discrete periods of arbitrarily-gated blocks/spawns/items/abilities/etc that unlock when you do certain things. This generally means you can't shortcut the game progression.
For example, creating a Wither might unlock the next Age, but to do that you have to brew a potion first to unlock the previous age. If you just jumped into the nether, went to a fortress, farmed wither skeletons until you got 3 heads, then grabbed some soul sand on your way back, when you build the wither it would simply not form into the boss.
Various strategies to execute the gating would include:
1)modifying worldgen...if you don't have the technology to mine iron ore yet, then iron ore itself would just not generate in the world. This would require retrogen functionality so that when you did unlock iron ore the game would go back and apply iron ore to the already-generated world.
2)visually hiding blocks you don't know about. For example, iron ore would initially generate like normal but if you haven't yet unlocked it the game would simply cover it up with some other block's texture (ie, an adjacent piece of smooth stone). It would still be iron ore and require a stone pick to harvest, you just wouldn't know if it was iron ore or not. Creative application of TNT or creepers would be required to obtain it at this point.
3)changing the block drop to something else. So, iron ore generates in the world like normal, you can clearly see it like normal, and you have the right tool to break it...but because you're in the wrong age and haven't actually unlocked the iron ore block it drops as cobble instead. There are two variations of this concept, one where the iron ore is just gone forever and you're stuck with "useless" cobblestone and one where it remains as iron ore for purposes of placement in the world but for smelting purposes it acts like cobble (so smelting a disguised piece of iron ore would not net you an iron ingot).
4)modifying the block-breaking rules. The iron ore generates normally, you can see it normally, you have the right tools to harvest it or in a pinch you know how to use tnt/creepers. Thing is, you're in the wrong age so even though these things are supposed to work they just don't. That iron ore block might be blast-resistant to explosions, it just breaks without a block drop entirely, or it's literally unbreakable for you.
Using DuhDerp's explanation of ages, I guess I can at least throw in my two cents about gating the game:
Story Time: I'm not a multiplayer kind of guy, but one time I thought I'd try a server with mods. The server was centralized around a modpack called Galactic Science, which used Hardcore Questing Mode to encourage the player to go through the modpack's story and progression. The bad thing about the server was that you were disallowed from trading things, and the only aspect of multiplayer left over was that people could group together to go through the progression together and possibly talk using the chat.
What was the point of that ramble, you might ask?
While an extreme example, I don't think Minecraft is a game that focusses on progression, especially in Multiplayer. The game has progression in Singleplayer, but it's incredibly quick. You can get iron in minutes and diamonds in at most a day of playtime. The core of Minecraft's progression is actually more about gathering a large number of blocks and items rather than high-quality items.
Now, that being said, it isn't as though quality items do not exist. Totem Poles, Elytra and even nicely enchanted armor are rare items that the player can find, but even these items are consumable. (Except the Elytra, although that requires a constant source of fireworks to be useful.)
It's also worth noting that even if you consider the dimensions in the game a form of progression as I do, there are very few items you need in high quantity in the Nether or End unless you need large amounts of Ender Pearls or Blaze Rods. The majority of progression items you need in quantity (Diamonds) are minable with a pickaxe you can get starting with nothing in only 30 minutes max.
In a multiplayer scenario, the player can easily go from Stone to Diamonds with the help of his friends, and the progression really comes into play when Diamonds are scarce or when you are off on an adventure and your pickaxe has broken.
Now, that being said, I do enjoy progression in the game when playing Singleplayer, and I frequently use mods to add more progression to the game, but I feel like my modded playtime and my vanilla playtime are very different. While playing modded I feel like I'm always busy trying to satisfy progression so I can get to some dimension and get the super-duper pickaxe. While playing vanilla I don't feel like I need to travel to the End, though I might do so in order to get a massive number of Ender Pearls or maybe get the Elytra. This is why I build lots of buildings in Vanilla, but rarely do I even feel like building a base in modded.
TL;DR: Progression can be fun when playing with mods, but it significantly changes the game and it tends to fall apart with multiplayer gameplay.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.
THX to DuhDerp for saving me the effort of confirming my guess as to what was meant.
Concur with C1ff: Strongly prefer not.
Leaving vanilla MC without a defined set of hoops through which one must jump (or at least a small set of quite flexible hoops that can be variously ordered) not only avoids putting-off all the players who want to 'mine' and 'craft' to the beat of their own drummer, but leaves a (comparatively) clear base on which all those who want a more constrained play experience/story arc can construct their edifices without first having to undo all the vanilla hoops.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.