Some TMNT stuff was always meant for grownups.
In the Mirage TMNT continuity, Leonardo of the future is a bodhisattva—a Buddhist holy man in the advanced stages before attaining buddhahood. He wields a carbon bokken, a gift from Donatello for his one-hundredth birthday, which produces bio-electricity based on his own willpower.
In Swan Song, Leo is revealed to have had a romantic relationship with Raven Shadowheart, also known as Radical. Radical's arch-nemesis, Complete Carnage, appeared from a brick wall one day whilst she and Leo were out for a walk, and snapped her neck, killing her.
Unable to find Carnage and avenge Radical's death, Leo joins a secret Buddhist monastery at the foot of Mount Fuji in Japan, where he trains for decades, eventually becoming a bodhisattva. Afterward, Leo decides to explore the world he had not seen in years.
While in San Francisco, Leonardo sees a news report of hostages being held in an impenetrable building of organic rock in Hong Kong, realizing that it may be Carnage. Leo had come to forgive Carnage, but when Leo sees the psychopath kill one of his hostages and taunt the turtle over Radical's murder, Leo's anger reignites and he kills Carnage. But these final acts of wrath and killing taint Leo's spirit, requiring him to rededicate himself to spiritual cleansing and compelling him to permanently forsake his katana.
Untold years after this incident, an even more elderly Leonardo is implied to be preparing himself for the end of his own lifespan, appearing to have finally achieved the buddhahood he had sought all his life.