There isn't really a definitive answer to this question as it depends on a number of factors, such as the child's age, maturity level and physical size. That said, most experts agree that bunk beds are generally not suitable for children under the age of six.
Bunk beds can be dangerous for young children as they are more likely to fall out of the top bunk. They may also struggle to climb up and down the ladder, which could lead to injuries. For these reasons, it's usually best to wait until your child is a bit older before investing in bunk beds.
Once your child reaches around six or seven years old, they should be able to safely use bunk beds. If you're still not sure, you can always ask your child's pediatrician for their opinion.