What does it mean by the youngest religion? Is it the oldest or most recent (used today) religion?

Young religions, also called New religious movements (NRMs) are religions created recently, examples like LaVeyan Satanism (1966), Thelema ( early 1900s), Latter Day Saints (LDS / Mormons, 1820s), Jehovah's Witnesses (JWs, late 1870s), Discordianism (1963)

Older religions include Judaism and Hinduism, etc

"Most recent" is difficult to answer, but the largest religion today is Christianity with over 2 billion followers, followed by Islam and then irreligion.