Battery

84% may seem healthy at first glance, but in reality your wife's iPhone 6s is nearing the end of the battery's useful life. You can schedule an appointment your Apple Store Genius Bar and have Apple put a new battery in her phone for $49.

See this Apple Support article about Battery Health --> iPhone Battery and Performance - Apple Support

The battery should definitely be replaced once it reaches 80% or less, and that phone is close to that.

If it were me, I'd more likely be looking at a newer more modern phone as it's possible iPhone 6s, got its last iOS upgrade with 14 this year.

What do the hollow bars mean?

If you mean the light blue bars they are apps with background usage, for example listening to a podcast with screen off.

length of bars represents time and not necessarily power consumed, so a long light blue bar may or may not be a high power user.

Then it's getting to be time to do so again. Batteries are consumable parts. And an iPhone 6s running the newest iOS versions on a device with limited (but just enough) RAM will push the battery harder than a modern newer phone with the latest chips in them. You need to realize you are dealing with a phone originally released in 2015. A lot has changed since then.

It has NOTHING to do with your age. 5 years however, is eons ago in terms of technology. A lot has happened in iPhone land in the last 5 years. And 5 years from now, a lot more will change and so on and so on.

Admitted I am only 70 but I can assure you 20 to 24 months is the expected li-ion battery life in a phone