A lot depends on the area and the community on how to portrait yourself, but we are both from the midwest so I think these tips should work for ya. Everything is appearance, timing, respect, and relating to the land owner. Age is nothing, as long as you portrait yourself as a mature individual (so no dumba$$ bumper stickers and decals on the vehicle either). Just put some casual clothes on that are clean, and a baseball cap will work.
So far the only times I've been turned down is because the land was leased or someone else (like family) has permission, "knock on wood." Part of that helps because I was born and raised in this area. Even though I don't know the land owner, I may know someone that lives down the road and so forth. That's why I always go to the house, state my full name, mention something (like name drop, or ask him if he knows so and so, then BS about it), and then state my intentions. Don't BS for much longer then a minute before you get your answer. You could end up running your mouth too long and get in to some bad territory.
The biggest thing is if you get a yes, regardless if you hunt or not, stop by within a week or right after a hunt and give an offering. I have about half dozen $25 Farm and Fleet gift cards in my center council for that reason. It doesn't have to be much, but something they will use and appreciate. I usually hand it to them in an envelope with a little note stating how much this means to you and what a passion the sport is to you. They can relate to that in some way. This will give you an easy in next year when you go back and say "Hey, do you remember me.... can I hunt again this year?"
Timing is everything too. Even if you aren't going to go out, the time to ask is during late season in the midwest. If you have a waterfowl season that extends past the deer season, you are golden! Midwesterners hold their whitetail hunting sacred. They don't want anything that will potentially ruin that hunt. After deer season, scout, hunt or whatever, but if you find a piece you would like to hunt, ask! you will have a lot of success. This is big because the next year you can go back earlier and do the "remember me" bit and you will have a better chance to get on there, now that they know you are not causing any trouble or harm to their deer hunting.
The most important thing is to respect the land!