Intermittent failure to go into "ready" mode

I have a 2012 Prius v, and recently I've had an issue where I follow my usual routine - get in, foot on brake, turn headlights on, buckle seatbelt, and then push the power button. All the accessories come on just fine... and then the car won't go to "ready" mode. After waiting for the accessories to turn off (pushing the power button again does nothing to speed this process up), I will try again, and the same thing happens. On the third try, it tends to go right to "ready" mode. I thought maybe the 12v battery was failing, so I had it checked and was told it was fine, and I'm stumped about why this keeps happening. The hybrid battery appears to be charging as normal while driving/braking. The failure to go to "ready" happens more often when the car is cold - while running errands around town I haven't had issues with it going to "ready" when the car is warm. Any idea what might be wrong? Thanks!

 

I had someone check them when it didn't go to ready the other day- brake lights were functioning as normal

 

I'm not sure. I bought the car used from an estate sale, and the lady selling it didn't know those details. I had the local toyota dealer do a PPI, and they didn't mention anything about the battery being old. I had it tested at Autozone, and they didn't try to sell me a new battery, so I guess I trust their judgement?

 

I looked at the 12v battery today and what I assume to be the manufacturing date (only date-like number I see) is 020911, meaning that the battery is ~8.5 years old? Maybe it's time for a new battery?