I started to feel the flutters around 16 weeks. I knew it was the kicking because I don't have gas issues or anything along those lines. When we went for our ultrasound at 17 weeks the tech said it was probably too early for me to feel them, but when the baby was up on the monitor he/she was kicking away like crazy! She said she wasn't surprised because of how much the baby was moving, and how responsive baby was.
Definitive movement not until about 20 weeks. By this I mean it was really easy to tell that baby was kicking. Now, at almost 25 weeks, we can see the kicks if baby kicks hard enough. It's almost bad enough that it can keep me up at night.