Why is my iPhone battery dying so fast?

I just bought an unopened iPhone 4S, because I prefer the smaller phones, and the battery drains in less than 5 hours. The phone was never used before, but is the age of the phone the reason it's dying so fast?

Will replacing the battery make a difference? As those are old as well even if I but an unopened one.

Added (1). bought*

Added (2). My previous iPhone 4S is just as old, but was used constantly. And that has much better battery. Lasted two days, even if being used a lot.

The brightness, if it is too high it will drain it faster, ad shut it off at night

Sounds like you need a new battery

No it won't make a difference. Those phones are clunky and the apps that we have now take up a lot of battery time so that's why it's best to buy the newer kinds of any phone, not just iPhones.

Apple no longer makes iPhone 4S, so either thats a used iPhone or it was in the box for a couple of years. I would say get the battery replaced. Or try ending apps that you don't use by double pressing the home button and turn off background app refresh in setting. Here is how to do that "Setting then general then Background app refresh, turn off". But I would highly recommend replacing the battery since it was just sitting there sealed without power.

Maybe brightness is on

You will see movie or play games that's why your battery dying so fast.

Because you probably play games, brightness is high and use the internet too much.

High brightness?

Charge with your device with original charger
& stop fast battery draining apps.