Hmm, tough to answer. So I'll break it down with your questions:
Do you actually change what kind of person you are depending on the subject being discussed?
Not really, to be honest. I still stand up for who I am, what I believe in, what's in my thoughts, et cetera. I have the same spirit here as I will anywhere else in the world, because that's how I've been raised, and how I've decided to be. I'm more free to be anyone I want and not be judged, but I'm still conscious about people paying attention to you. I have gotten really nice PMs/messages on my profile (that you can read yourself) on about who I am, and people are respecting me for that. I may not know them personally, but it's pleasing to know that even concealed behind a computer screen, people are watching me (or in this case, reading me lol), and thus I need to be the example I am.
Do you lie about your age, or other facts about yourself?
Back in the early days of my surfing the web and becoming members on certain sites, yes, I used to. As I've gotten older, I've become more mild (which I suppose isn't such a good idea heh) and more willing to give my age and some facts, usually facts that are about the general me, nothing very personal (though there are rare exceptions). And because now that people know my age, I should ACT my age and not be such a n00b like I have been in the past and less annoying.
Are you a more confidant person online?
You could say that. I'm more afraid to speak my mind in a person's face than just in words, but I still do say what's on my mind either way, it's just more detailed on the Internet. Yes, I'm still judged on what I put, but I feel more confident about who I am and why I believe the way I believe, and it makes things a little bit more easier to do in real life. Recently, even though they're my friends, I'm finding it smooth to tell them almost everything that's on my mind. Whether it's about my political views (when it comes up, though, most happen to share the same insight, lol), being a Mormon or whatever, it's not as difficult to do as, say, a year ago. Maturity, I believe, plays a key role here, and I've learned to grow up, most from interacting with people here on Serebii.
Do you act completely different online than you would anywhere else?
Naw, I don't believe I do. I'm more free-going here than in a place where you're more likely to get yelled at just for being too hyperactive and talkative and unable to get your point across. You could say I have a louder "voice" on the Internet than I do in the real world.