The Southwest rule is preboarding for 5 and under. These folks know quite a bit about turning a plane around quickly, so I think they've probably found the most efficient age cut off in terms of on time perfomance.

Until our kids were two, we had to deal with a baby/toddler car/airplance seat. Those things are heavy and awkward. I would be carrying that plus some other luggage. You don't want folks hauling this stuff down a narrow aisle while other passengers are in their aisle seats.

Even with 2-5 years, you're dealing with lots of meltdown potential. If that happens while everyone is lined up in the jetway or the aisle, it will be slow going for everyone. I say get the five and unders in early, and get 'em seated. If they freak out, let it be while they are in their seats.