My son is 4 months old, and a bit tall for his age. I purchased this in a 6 month size to get us through the winter. So it's a little big on him now, but it keeps him toasty and warm. It's very easy to put him in it, and it's not too bulky in the carseat. I love that it covers his hands and feet, Minnesota winters CAN get pretty cold and this little suit is perfect!