The only way to do it without mods is turn off story progression in your game options. This will stop them from having babies and getting married but I think they will still adopt pets and unemployed sims will get jobs (but won't be promoted).
If you want to go back to your sims exactly as you left them, you'll need a mod. There are a few available I think but I use twallan's story progression mod, you have to tweak the options a little to get it how you want it. Full instructions and the file to download are on his page at
https://nraas.wikispaces.com/StoryProgressionRead the page, do a little googling to learn more.
Hope that helps
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