Youthful dream meaning is explained below using 2 different sources. Each interpretation is shown with its source.

Interpretation 1 — Stephen Girard

For an aged person to dream of being again youthful and active, is an indication that death will soon call the dreamer from this world. 
Source: The Mystic Dream Book Author: Stephen Girard

Interpretation 2 — Margaret Hamilton

Figuratively immature
Source: Dream Dictionary Unlimited Author: Margaret Hamilton