Age of Empires II: The Age of Kings
(preferably The Conquerors expansion) all the way. I was fine but the graphics and dynamics are a bit off and II tweaked that. III was not the same game, not by a mile, so I'd stay away. Age of Empires II has a free HD patch for the retail version which also adds some nice features. The patch also fixes LAN play which is difficult to use otherwise. I'd advise staying away from the Steam version as it had some serious issues, doesn't have LAN play, and the retail version works without issue on Win 7. With Age of Kings LAN play only one player (out of a maximum of
needs to have a CD in the drive to play. The resource management in Age of Empires is none too intense, it is a real-time game, and while a player can micro-manage he doesn't have to in order to play.
I'm confident that your 8 year old can manage this game as I was playing it when I was about the same age (as well as Red Alert: C&C I lol).
Of course, none of the Age of Empires will give you tanks and jets.
If you want tanks and jets I suggest the first Empire Earth
(available DRM free at GOG.com) and its expansion. The retail version had some issues with win7. Empire Earth is made by many of the people who made Age of Empires and allows RTS gamers to battle in any subset of times from the time of cavemen through a robotic-filled future. This includes jet and tank laden eras like WW1, WW2 and "modern". This game also has non-intensive resource management. In fact, this has some of the easiest resource management as resources almost never run out: the stone/gold/iron mines would take literally days of gameplay to exhaust. This is a somewhat radical shift in RTS gameplay. This game uses blocky 3d graphics (which aren't that bad when zoomed out; one of the gimmicks of the game is that players can "zoom in to eye level" with their soldiers) but has native HD support. However when playing the game you might want to "zoom out" further than the game allows. I have not found an answer to this but it is not game breaking.
Also, Age of Mythology
is fun. No jets, but it has fighting robots and the ability to call down lightning from the heavens which might be as fun.
I'd recommend staying away from Steam. There are a slew of RTSes that don't use it and Steam is not only a hassle as it requires buying multiple copies of the games but many games I've found (perhaps because it circumvents having to buy multiple copies) do not have true LAN play: too many Steam games require you to have a connection to Valve in order to play games between 2 computers in the same house. I just don't see the point in that.