What is the name of the waxy covering on the epidermis of a young stem?

The waxy covering on the epidermis of a young stem is known as the cuticle.

The cuticle is a waxy layer that covers the epidermis of many aerial plant organs, including stems, leaves, and fruits. Its primary function is to protect the plant from desiccation (water loss) by reducing transpiration, which is the loss of water vapor through the plant's stomata. The cuticle is composed of a mixture of lipids, including waxes and cutin, and can vary in thickness depending on the plant species and environmental conditions. 

Suberin is a type of lipid found in the cell walls of cork tissue and is part of the periderm, which is a secondary protective tissue that replaces the epidermis in older stems and roots. Latex is a milky sap that is produced by some plants and can have various functions, such as defense against herbivores or sealing wounds.