Junior wetsuits go from Age 1 to Age 16, at the upper end some of the age 16 wetsuits will fit up to a ladies size 10 with a height of 5'5".
There are a variety of styles, thicknesses and entry systems in the wetsuits. Chest Zip is the warmest style of entry system as long as the neck of the suit is tight to the neck, back zip is the most common standard entry on wetsuits, some toddler suits come with a front zip that runs vertical rather than across the chest.
3/2mm is the standard summer weight wetsuit typically suitable for June to September.
4/3mm we call spring wetsuits and suit for those that want to stay in the water longer and get a longer season, May to October, or through the winter for swimmers.
5/4mm winter wetsuits, typically only for surfers or those not moving around in the water very much, they can be too thick or buoyant for swimmers unless you are in particularly cold water.