Well. My oh my. This was absolutely GORGEOUS. I can't tell you how intense it felt, with all that adolescent angst, and how funny those moments of relief and comedy were. I mean, you really brought out the manic highs and depressive lows that no one ever forgets from teenagedness. I seriously felt a little crazy, for moments there. The Insanity? Augh. The ninety galleon dragon? Augh. The kissing and the moment when Harry doubts because Severus said 'like' but he didn't say 'like like'?? Augh! And THEN! And then the mirror of Erised and Harry reaching out like all his dreams have come true and the written-under-the-influence-yet-precisely-crafted letter!! Augh augh augh!!! (Fantastic touch, btw, in not showing us the full contents of that letter. Fantastic.)
I was really amazed at how deftly you worked the de-ageing and re-ageing angle so smoothly. I appreciated the absence of complex magical theory and explanations because honestly, I personally want to get to the good parts, where there's secret longing and angry self-beratement and private teeth-gnashing--and you brought it. Completely. I've never been a particular fan of time-turner or de-ageing stories but this fic highlights their appeal in such a vivid, alluring manner. Some aspects of the de-ageing theme in general might strike me as somewhat difficult--but you handled them beautifully here. If suddenly-teen!Snape's dialogue was a little wooden, well, it only enhances his out-of-place condition. If re-aged!Snape's apologies to Harry were unusually lavish, it only increases a reader's (or at least, my!) sympathy for, and empathy with, Harry, as he wishes and wishes he could see his eighteen year old friend again.
I have so much love for this fic and your characters. (Harry's 'um's were quite charming.) Our team is SO going to win. :D ♥