Yesterday, my Dually - who goes by the name Mark, but that's another story - wouldn't turn over.
I'd left its battery tender off for a couple weeks or so.
Gave it a quick charge, and in 15-20 minutes it lit right off.

Looked at what appeared to be a date sticker - it said "5/11" - and looked in its vehicle file.
It's a 3-year battery, bought June 2011.
Sure enough, the battery had entered puberty !
That beats my previous record of 11 years.

Needless to say, even though the old battery has maintained 12.3 V for a couple days just sitting disconnected, I bought a new one today.