The oldest save I have is the one I started with. I got the game at Christmas time and it was basically my playtest save, because I never played any Sims games before I got Sims 4. I don't play that original save anymore, because there's so much HISTORY there, I learned so much in that save about how to play the game.
The first big lesson I learned in that save: I accidentally aged a sim up prematurely because I didn't know aging could happen autonomously, so he died way before his wife did. I was still a newbie at the game and didn't know I could save households in CAS so easily, otherwise I would've saved him. Ryland the Random Townie is the only one for my precious first sim, Jenny Tibbs. I'll never be able to get him back, and I don't think I can remake him using pictures :cry: I don't even remember his traits! His legacy lives on, though, through his EXTENSIVE list of children, grandchildren, and great-grandchildren, even great-GREAT-grandchildren! They live all over Willow Creek, and have even moved into part of Oasis Springs. If I played that save again, there's no doubt in my mind that they'd spread like a plague into the other worlds, these guys breed so much.
I have another family in that save that took over Oasis Springs like the Tibbs took over Willow Creek (they even inter married at some point.) I had the criminal mom take care of her teen daughter, the daughter eventually marrying Alexander Goth. I didn't know how big of a deal the Goths were at the time. Mortimer was dead, and Bella and Cassandra were elders, so I ended up killing/deleting them.
My next oldest save was started when Get To Work came out, purely to play with that expansion.
3rd oldest is my Legacy save, which is the oldest one I currently play. I started it before Get Together came out, and my founder (my self-sim who ended up marrying Bella Goth) ended up dying in Windenburg after painting one final painting there. I made sure to stop playing until the expansion came out so he could die seeing a new world (right in front of the fountain by the café! He made a beautiful painting too!)
I have a lot more other saves that I play, but my legacy save (all of my saves are legacies, really, lol) is the oldest I still play.