The 'Deathstroke: Rebirth' series is also great. It delves deeper into his backstory, his family relationships, especially his relationship with his son. His skills as a mercenary are also highlighted in a very engaging way throughout this story. There are intense fight scenes and moral dilemmas that make it a top Deathstroke story.
Deathstroke's age in the comics is a bit fluid. It often depends on the creative direction of the story. Usually, he's thought to be in his mid-40s to early 50s.
Well, one possibility could be that Naruto, influenced by Kurama's knowledge or some strange magic, starts to admire Deathstroke's combat skills. So he might try to incorporate some of Deathstroke's fighting tactics into his own ninja style. For example, Deathstroke is known for his precise and calculated strikes, which Naruto could adapt to his Rasengan attacks to make them more effective.
One great 'Deathstroke Fanfiction' is 'The Redemption of Deathstroke'. It shows a different side of Deathstroke, exploring his possible path towards redemption. It has some great character development and action scenes that really draw you in.