I used an infant seat first with both of mine, then a rear-facing convertible when they outgrew it. I turned my first around forward-facing at about 18 months, only because the back seat of the car we were driving wasn't big enough for the convertible seat we had. But when he was 3.5, he started wanting to sit in his sister's rear-facing seat, and he was still within the weight/height limits, so when we had to get a new car and car seats, we just put them both rear-facing. My oldest will be 4 in a couple of weeks, and my second just turned 2 last month, and both are rear-facing.
General recommendation for forward-facing, based on safety info, is when they outgrow the rear-facing limits of the seats you have. Most convertible seats go up to at least 35 lbs rear-facing, and they're too tall when the top of their head is within in inch of the top of the hard shell.
at 9:22 AM on May. 27, 2011