Hello! Multiple factors could be affecting your Apple Watch's battery life, such as Bluetooth and how active you are. Read the 'Tips for Apple Watch' section in this article to find out how to preserve your watch's battery throughout the day: Batteries - Maximizing Performance - Apple.
As you mentioned, depending on when you bought your Apple Watch, its battery could be ageing. This makes the battery less capable of holding a charge. You can check your watch's battery health by following these steps:
- On your watch, open Settings.
- Tap Battery > Battery Health.
If the maximum capacity is less than 80%, it's recommended that you get the battery replaced: https://getsupport.apple.com
Take care!