Unlike in the Sims 2, each save file exists in its own, separate universe.
In the Sims 2, time did not pass for sims you weren't playing. However, in the Sims 3, the "Story Progression" means that no matter which household you play, time will pass for all families in the neighborhood.
The one thing that is shared between neighborhoods is the Family bins -- that is to say, both you and your fiancee should have each other's sim in their neighborhood bin. If you then move this sim into the neighborhood (even if you don't change the active household), you will be able to find, meet, seduce, and eventually marry each other's sim.
However, because you have separate save files, marrying in one neighborhood won't simultaneously marry the sims in the second neighborhood.
It is possible to simply play multiple active families in the same neighborhood, but since every household ages at the same rate in The Sims 3, this will probably not be a preferable way to play (not to mention losing wants every time you switch active households).
Making a Sim "Younger"
Life Fruit
You can reduce a Sim's age by a single day by eating a raw Life Fruit. However, they can only be planted by a level 7 Gardener and the "Unknown Special Seeds" or "Life Seeds" are only available under certain circumstances:
Sims can receive the seed by searching the town, as a science career reward, by exploring the catacombs in a mausoleum, fishing it out of a body of water, or while using the Time Machine.
Source: Sims 3 Wikia, Life Fruit page
Ambrosia
The better option is to cook Ambrosia, which resets the Sims age to a predefined value (adults eating it will reset to have 26 days until aging). It can also bring Ghost Sims back to life.
Ambrosia requires purchasing the recipe (§12000), a level 10 Cooking Skill and then one Life Fruit and one Deathfish as ingredients.
A deathfish can only be caught with level 10 Fishing skill, from the Graveyard between midnight and 5AM and needs angelfish bait. Note that if you catch enough deathfish you can stock a pond and have a supply of these available all day round.
Young Again potion
This potion requires the Generations pack and is created using a chemistry set, it resets a Sim who is older than a young adult back to day 1 of the young adult.
You will need a high Logic skill for this.
Prevent / slow down further aging
Age Freeze Potion
Another potion, so also requires the Generations pack. This is a lifetime reward that requires 65000 lifetime happiness to obtain. Drinking it will completely stop your Sim from aging further. Note that if you eat birthday cake after drinking the potion, your aging will start up again.
Change form
If you have the relevant expansion pack, you could also get the Sim converted to a Vampire or Mummy, as they have longer life spans.
Cheats
If you enable the "god mode" cheat (testingcheatsenabled true) in the cheat console, which is activated by pressing control, shift and c in unison, you can decrease the sim's age, but only in the given stage of life. By shift clicking the sim, and editing them (you don't even have to change anything) in create a sim, the game will set their life back to the first day of whatever stage they are in.
Best Answer
I'm guessing the game has been hard coded to use C:\ instead of checking where the documents directory is, you could use a NTFS Symbolic Link to workaround it. This will essentially create a shortcut, that programs will actually follow it properly.
To create one you will need to open a command prompt as admin. To do that just open the Start menu, type "cmd" and press Control + Shift + Enter. From there the basic command your going to want is
Of course supplement the directories for the ones you're using. You can either do it for the entire Documents directory or just for The Sims 3 data. In other words, move the Sims 3 folder, for example, to drive D before entering the above command.