Sound is only coming out of one earbud?

My old apple headphones that I use all the time (might) not work. When I was watching youtube on my iphone, the headphones were just fine- the sound was coming out on both sides without a problem. When I watched a different video, the sound was only emitting from the left earbud. I tried using the headphones on other devices (my laptop, my moms ipad, my ipod, etc.) the problem still persists. The headphones are really old, at least 9 years old (came with my dad's old iPod mini). Could the age be a problem? I don't remember ever misusing the headphones…

Your question confuses me a bit. Are you watching the same video on different devices with the same result? If so, then the audio from the video is not stereo. If sound is only being produced by the left side then the right side has probably stopped working. Age and build quality of your headphones could definitely be the issue here. However, if they are at least 9 years old, I think that your Apple headphones served you well.