Author's Notes: A snippet written loads of years ago (in 2006, I think), but for some reason I never posted it here at the time. It's non-canon-compliant from at least HBP onwards.
Harry tries to tell Severus that his age doesn't matter. He tries just as hard to remind Severus that his looks never mattered to Harry.
Severus still won't let go of the idea that Harry is looking for something better; someone his own age, who is handsome enough to intrigue him in a way that Severus thinks he can't. Whenever Draco visits, Severus insists on acting so possessive and jealous that the whole thing becomes uncomfortable. It's not normal behaviour for him, and Harry is slightly worried, though he doesn't admit as much.
Harry tells Draco (and himself) that Severus only thinks the two of them are having an affair because Severus himself is feeling insecure. That will pass soon enough, Harry thinks, for Severus is not by his nature an insecure man.
He's jealous about nothing, and he'll realise that soon enough.
He doesn't, though.
Harry recognizes, as Draco feeds him enough alcohol to numb the pain of how final his earlier confrontation with Severus seemed, that Severus's looks and age do matter. It's irrelevant, in the end, that they only matter to Severus himself.
~FIN~