Oh man, you guys are chumps. CHUMPS. Almost none of you even tried to stop 9/11 or prove that you were a time traveler. It was all "Oh, I'll actually have a real childhood" or some bullshit like that.
Here's what you do: Act natural at first. Do NOTHING at all to change the timeline except contact some FBI agent or something and explain to them that you're from the future. There should be a variety of ways to convincingly do that if you have even the memory of a 5 year old. For example, did you play video games as a kid? Did you play them well enough to know what will be in, say, an unreleased Japanese game about pocket monsters? Why don't you call JK Rowling up and tell her the ending of her own story, which she says she knew even as she wrote the first book? It will take a couple of tries, but combined with your (for a 5 year old) supergenius level intellect you should come off as extremely convincing.
Once you've proved things conclusively, someone will probably lock you away to make sure your predictions remain as accurate as possible. This is for the best. It won't be perfect, of course, because you not living your normal life will alter the timeline on its own, but it will be close. There is one thing you never do, however: Don't tell anyone what your original age was when you were sent back in time.
Over a few years, your predictions will have saved countless lives and improved many others. Depending on the time period, you can stop 9/11, get New Orleans ready for Katrina, send a SWAT team to prevent the Jonestown massacre, and advance any field of study in which you have adequate knowledge.
But that's not where you'll do the most good. No, the most good you'll do will come from lying about the future.
Remember how I said not to tell anyone what your age was before going back in time? See, there are a lot of things you won't be able to prove, but can lie as if you're able to prove them once you've gathered enough credibility. Once you become internationally famous, you'll be able to bring down totalitarian regimes by personally contacting their leader with threats of what happens to them if they don't peaceably dismantle their government and form a democracy.
But most importantly of all, you'll be able to call George Lucas and tell him to let someone else make the Star Wars prequels. Once you've done that, your purpose in going back in time will have been achieved and you will return to your original body.