Peter thinks of his father sometimes. (In all his travels, he has never met a being more alien than his own damn father.)
He remembers Ego's words on immortality. He looks around himself, at his team, his family, and thinks that it is a good thing that the Light is gone and he is mortal. He doesn't want to remain young while they grow old and die.
(One day, he looks into a mirror, sees a grey hair and feels only relief.)
When age spots appear on his hands his teammates are alarmed. Apparently it's a Terran thing, because he remembers his grandfather having them. He asked about it once, and was told it has something to do with age. He welcomes them with a sort of grateful wonder.
They grow old together (not Groot. He grows up, but doesn't age like they do. His species is a long lived one.) They start healing slower when they get hurt and Peter meets Groot's eyes and sees sadness, the awful knowledge that Groot will remain alone when they are gone.
(When Peter looks away, Groot looks even sadder. He looks sadder.)
The age slows their bodies down, but experience matters. They adapt, they continue saving the galaxy like the badasses they are.
Rocket is the first to go, then Gamora, then Drax. It is sad, but their friends have lived long, fulfilled lives. And grief doesn't relinquish a person easily, but it's- Drax had laughed, a full bodied laugh, right before he died. He was not afraid, just curious, and strangely delighted.
After Drax's funeral they go home. Peter wishes Groot a good night and goes to sleep. It takes him awhile to get to his bedroom. He counts every torturously slow step as a victory. His knees twinge when he sits down on the bed. He puts his cane next to the nightstand and slowly lowers himself down. His back always hurts. Medicine can only help so much. He feels too tired to change, so he just covers himself with a blanket and turns on his side, trying to find a comfortable position. He will wake up in a lot of pain, he knows. He wishes he could wake up and not hurt.
He wakes up and doesn't hurt.
It is wonderful and strange and wrong. Alarm bells go off in his mind and he jumps out of bed (oh God, he jumps), and runs (God, no) to the bathroom and the mirror lies (it must it must it must) for he looks as old as he did when he met Ego.
Peter wished he could grow old, and he did. He wished not to be in pain, and he was not. His father created an entire planet around himself, what are a few wrinkles compared to that?
Ego's light may not be alive anymore, but Peter's is.
(One does not simply stop being a Celestial (or half) just because one's father dies.)
He wishes for his family to be alive. (They are not.)
It takes him a long time to leave the bathroom.
When he sees Groot, he wishes him a good morning in a choked voice and Groot looks at him, solemn and unsurprised.
"I am Groot," he says, "I am Groot."
"Me too." Peter answers. Groot grows a branch towards him, and around him, and pulls him close, into an embrace. Peter remembers Groot when he was a baby, still in his pot; remembers the tiny, adorable, vicious child and the sullen teenager; remembers the moment he realized that Groot was not a child anymore and when he went on his first job alone.
(He realizes that he will live long enough to see Groot die as well.)
He remembers infinity, seen through his father's eyes. It had no age spots.
He looks down at his hands. They are unmarked.
He tenses, and Groot tightens his arm around him and whispers, gently, so unbearably gently, "I am Groot. I am Groot."