It's probable Liu Kang (or any other fighter) had minute special powers before being MK champ, they were just intensified after defeating Shang Tsung (in MK1). Don't know if it's granted but the Dragon Medallion does something similar to this.

Yes like age at a much slower rate.

Ah you see that makes sense. Thank you.

Was it MK Conquest that first explicitly stated that the champion won't age until the next tournament, or had a game's lore said that previously?

Maybe both. The overthrown board of the MK1 tournament or Master Wu (my guess founder of Wu Shi Academy) initiated the champion would not age for roughly half a century.

The reigning Mortal Kombat Champion does not age until he gets defeated in Mortal Kombat. The original Kung Lao beat Shang Tsung and became the champion and didn't age for 50 years before Goro defeated and killed him in the tournament. That's the only ability that I believe being the champion does for you.