Why does there have to be visible lines between one age and another? If you have a research tree spanning the entire timeline you wish to portray, then you get the same effect without the race to achieve a higher age. What I'm suggesting is a more gradual approach to research. For example, in AOE2, you can not build Cannon Galleons until you reach the Imperial Age. An alternative method would be to have upgrades at your Dock/Shipyard for Shipbuilding techniques. Perhaps when you can first build Shipyards, the only vessel you can create is a rowing boat. After researching Sails at the Shipyard, and a Ropemaking technology elsewhere, fishing boats could be constructed. After a few more Shipbuilding upgrades and a Gunpowder technology at an Alchemist's workshop, you would finally be able to construct your cannon-equipped warships.

The technology tree would be slightly harder to learn, as there would be more things to research, and more complex prerequisites due to the fact that you can not simply say 'This does not become available until the Iron Age'. Not only does this add some degree of realism, but it also allows the player to focus on the technologies that they want, instead of paying a fortune to advance to a new age, where they will only need half the technology they invested in.

The problem of whether advancing to the next age or advancing within the current age is eliminated, because it is the act of advancing within the current age that allows you to progress to the more advanced technologies.

You could also motivate players to take part in combat early by implementing a tech-stealing method as Iron Chef Carnage suggested. Having Spy units specifically for this purpose is one method. Also, invading an opponent's settlement could have the added bonus of gaining insight to their technologies (perhaps reducing costs/time to research when you research the same technology, or having a chance of learning it right away).

the problem with all of these games is that buildings have too many hp and that units move too slowly. Combat is too cumbersome. All you need to do is get good (and I mean really good) at Starcraft, which is the pinnacle of RTS design, and use it as your example.

Actually, that was what always cheesed me off about StarCraft: You can't steal tech. You occupy a Protoss base with goliaths and tanks and a science vessel or two, and you can't juice up your rifles with plasma, or put shields on your marines, or strap scarabs to your spider mines. All you can do is demolish everything and move on. What a drag. Even with the same race, I can't use a green factory to build red tanks. I have to burn it down and spend my own cash to build one that's just like it.

If your own tech can become a liability, then tech-rushing loses some of its appeal. You won't race to get nukes if a nuke would rock your world, and so a balance of offensive and defenseive capability fends of the apocalyptic battle that every StarCraft game over fifteen minutes long ends with.