Everyone ages cognitively at a different rate, but unless you're approaching 70, which I doubt, that loss of ability to concentrate you're experiencing is unlikely to be physical. If you're under 30 and you're feeling you've already lost significant ability to concentrate you probably want to do a deep rethink on how you allow web/mobile/social media/etc to impact you. It's hard to cut back, because those distractions are an actual addiction.