2005 Battery worries at 136,000?

Yes you should worry.
HV Batteries last 10-15 years, 12v 4-6 years. HV Batteries can be replaced for a low as $1600 if DIY. 12v batteries cost $150-250 if DIY.
Use the search function, there are some very good discussions on this topic.

 

I have to say, years seems more a concern than mileage. But your Prius is up there in years.

Heat can shorten HV battery life, if the car was always a Western Washington car, it will not have seen much heat. To be honest Seattle does not have much of a 'Winter' either. Living close to the ocean, you may wish to inspect for corrosion.

If the previous owner(s) had pets, you may wish to clean the battery ventilation system.

Pictures of a dirty fan Prius battery cooling fan cleaning, P0A82 - Luscious Garage | Hybrid Specialists

Crude video showing how to remove the fan

 

congrats and welcome to priuschat!

no point in worrying, but time to start investigating replacements while you're not under the gun.

all the best!

 

My guess is that 90% of 2005s are still working fine on their first batteries. But that is a total guess. Is there any definitive data on the lifetimes of Prius and/or their batteries?

 

I think Consumer Reports said 1/2% failed each year, which would imply about 8% failure so far. But that is also not definitive. As Toyota will not know about out of warranty work, I doubt there is a better source.