I think I need a new battery, its six years old, reading 12.5 volts, big crank, but the starter clicks or turns slow. Probably need a new battery?

More important than what the batter reads at rest - is what it reads while the starter is running and placing a load on the battery.

Lead Acid wet well batteries often give you a warning when they are getting old and used up - they slowly lose their capacity.  The starter turns slower and for shorter periods of time......then one day they just can't do it anymore.

Absorbed Glass Mat (AGM) batteries are less forgiving most of the time.  They work fine for years - then all of the sudden they are done.  This failure can happen as you get ready to head out on a ride - or it can happen when you are out on a ride and a long way from home.  When they fail you likely can ride home if you get the bike jumped - make sure the jumping battery does not have the engine running as the alternator can ruin your ignition system.

I have no idea how/when Lithium batteries fail,

Thanks guys, I ordered a new big crank. I believe  you are right!

Thanks, iI checked my records,  it's  actually  7 years old.

Good idea, It took me by surprise  the wife and I were all dressed to go for a ride. No warning signs. It just quit.

Put the new battery in. Works good. FYI, autozone gave me a $10 dollar credit for the core.Thanks for the help!

Big crank, but when I recycled the dead battery at autozone, they gave me a $10 credit. I  got a can of seafoam with it! Thanks.