If an object was stolen in one's childhood or teenage years, without knowledge of the laws regarding stealing (or not caring), if this object is impossible to return, either because the person has died or it's somewhere you can't remember etc, what is done with the object? Is it thrown away? What is the method of doing teshuva for a situation like this?
Rabbi Moshe Feinstein has a responsum to someone who's trying to repent for all sorts of stuff stolen long ago.
If it's impossible (or even unfeasible) to return the actual item, then just its value should be repaid to the victim. The Talmud's example here is if I know I stole this beam from Mr. Smith, but I've now built a mansion centered on this beam. The strict principle of expungement would demand I tear down the mansion to get to this beam, but rabbinic policy wanted to give people a reasonable chance at repentance -- hence, just pay Mr. Smith the value of the beam.
Rabbi Feinstein addresses questions of stealing from a known fund or organization, if decades later it has a different name, purpose, board, or the like; however, if there's no way to identify the recipient, then instead the money should be put into something of the public good. It has to be the public, not just the poor. (Assuming you didn't just steal from the poor.) The Talmud's example is to build public wells, and that way it's likely that your victims will at some point avail themselves of them. Rabbi Feinstein's recommendation (in 1950s America) was actually mikvah funds, a nifty twist on the Talmud's "water systems." As I really doubt your local water utility would accept a donation, I would wonder if something like a volunteer fire department or a library might work too.