My oldest son's 19 -- now practically an adult. For the past three years, he's been going out with a girl (16) who seems to like him a lot, whom he seems to like as much and whom he gets along with fairly well. It wouldn't surprise me if they decided to stay together for long time -- like moving in together or anything alike.
The idea, worry and fear of becoming a grandfather at my age (39) worries me in deed. I hope he doesn't make the same mistakes I made growing up. Having him when I was 20 was hard (financially especially), yet never regretted having him. I'm proud of my boy.
Of course, there are difference between us. He's into sports and not a geek like me. He's also has a strong Catholic upbringing. Most importantly he doesn't seem to be mentally unstable as I am though having the short fuse temper that runs in the family.
At the same time, we've got similar taste in music and I've tried teaching him about music instruments (guitars, drums, etc) and music in general from the 1960's and up.