to be more specific, you can go to the scenario options (may not be the exact name, could be scenario properties or something like that) and it is easier to give players techs so you could easily give everyone all techs from one or more ages. in this same section you could also start players with different amounts of various units, set the amount of starting treasury and other things including the number of turns (there are other ways to limit the number of turns including the game set up screen by altering the game limits)
you can then go to the edit technologies section in the editor and delete all the techs from the ages you do not want to play, i am not sure if you can just go to an era and delete a whole era but i don't think so. for example, if youi want to start in the middle ages and end in the industrial age you wold give everyone all the ancient age techs, then delete all the modern age techs, and some of the industrial age techs if you did not want to play all the industrial age. you could also put all the techs you wanted into one era, but you would also need to edit the x,y coordinates for the science advisor, set prerequisites for the newly moved techs, and optionally adjust the arrows to reflect prerequisites
if you generate a map, you can place cities and units and tile improvements (irrigation, roads, mines, rails, etc) you can also set the size of the city, its culture, and any improvements in the city. you have to "set the active player" before placing cities / units.
basically, if you get well acquainted with the editor you can create a scenario on par with the built in scenarios, but it can be a lot of work and some text changes might be necessary, you can find a lot of help in the creations and customizations forum, but all this may be more information than you wanted
