I really relaxed my characterization and attention to canon detail in my last two Snarries (I think it was in reaction to the Snaco I wrote - it tore me apart and I just wanted 'light' afterward). Kreacher not only had wellies, he had a nightcap at one point, and I completely blew it when I had Hermione find a title to the Fidelius-protected property filed at the Ministry. *forehead slap* There were several other goofs that I can't recall right now, but that I found after I'd turned it in. Definitely not one of my more canon-compliant stories. *g*
I debated about the sex. Being in my mid-forties, I was pretty comfortable with the Harry side of things, but I had no idea about how Snape would experience sex for the first time at 70. I finally figured a) Gingertart likes virgin!Snape, b) whenever I first begin having sex with someone I tend to be quite enthusiastic, which could explain the frequency, and c) wizards age differently than Muggles, so might well have a reasonably high sex drive well past what one would normally imagine.
(Though one of my partner's mother had a 70-year-old lover, and they seemed to take a lot of showers together whenever we visited for the weekend. ;-D)