It's recommended that you rear face your child up to 2 years old. In the event of an accident, a rear facing infant will be supported by the shell of the car seat. In a rear facing car seat, the shell of the car seat will stop the child from moving. In a forward facing car seat, the harness will stop the child from moving, placing strain on the child's neck and spine. This is especially pronounced for babies and toddlers, as their heads are quite heavy in relation to their body. Even if your toddler is 15kgs, they're not quite ready for a booster yet.
If your child is happy rear facing at two years old and you have a car seat that is suitable for their height and weight, there is no need for you to turn them around to forward facing. This is quite popular in some parts of Europe, and is called Extended Rear Facing or ERF. Below you'll find some options for both forward and rear facing car seats for two year olds.