How far does a criminal background check go back in Indiana?

The dawn of time. Well, probably, not quite that far, only back to say 1816, when Indiana became a state.

You should generally expect a criminal background check to go back as far as the person making it wants it to and is willing to pay for (it's easier and cheaper to check the last 7 or 10 years than it is to go back to 1816). There's no automatic "oh, it's been three years, that falls off your record" kind of rule in effect that applies to everything.

If someone is doing a background check on you, it's probably in your best interest to tell them ahead of time what they're going to find. The fact you did something stupid when you were 18 is less likely to be important to potential employers than whether or not you tried to hide it from them; telling them - or not - says something about what you're like now and they willregard that as relevant.