In Godzilla vs. Kong, Kong's appearance has matured significantly since Kong: Skull Island, reflecting his journey from adolescence to a seasoned titan, but it has led to speculation over how old Kong is. Despite being younger than many MonsterVerse titans, his weathered visage and demeanor speak to a life of hardship and survival. Previously depicted as the last of his kind battling Skullcrawlers in the 1970s in Kong: Skull Island, Kong has grown, both physically and experientially, bearing the scars of past conflicts and the wisdom gained from them.
https://screenrant.com/godzilla-vs-kong-old-age-beard-explained/