This is fantastic! You've written the boys without so much of their emotional baggage, and allowed their similarities--abusive childhoods, isolation, and destiny--to forge strong connective bonds instead of ties of anger, fear, pride, and resentment.
Great characterizations! I adore your young, vulnerable, self-aware Severus! Harry actually giving himself permission to let go of his mistrust of Severus could only happen when Snape is not continually feeding it, but this is a mature boy looking past the quick, safe, unthinking responses. Your Dumbledore is even more ambiguous than canon, it would be interesting to see his reactions further in the story.
Speaking of which, at first I felt cheated of my denouement at your ending, but upon reflection, I suppose I do like the fact that I can use my imagination to supply the rest of the story that I like! (I'm going for Severus being re-aged, and the two continuing the relationship, yummy!)
And speaking of yummy, the sex was wondrous. Two inexperienced boys discovering each other can be heart-breakingly beautiful, and you've so captured that. Their lovemaking was erotic, sweet, and delicious!