Short answer: it doesn't (matter, that is). There are trustworthy and untrustworthy people all over the world. Just because they're online doesn't mean they're the latter, and just because they're in 'real life' doesn't mean they're the former. In either case, it's important to honor your own instincts and with online stuff be extra cautious because you can't see them, be around them, etc.
I have had long-term friendships and relationships online. I am still friends with several people I used to chat with (I don't chat anymore) over ten years ago. I care for them and think about them. I have met one in person, finally, the year before last. I have never met the other one. But I don't trust him any less than the one I've met.
Hope that helps. The world is a very different place now than it used to be. There are still all kinds of creeps and weirdos online. But they're out in the world in everyday life too. So there's really not much of a difference. If you trust them, that's all that matters. I mean, you've been talking to this person for almost twenty years. That's a long-*** time.