Are you sure it's because you are getting older, and not because of another factor like technology? There is growing evidence that the constant consumption of information e.g. on social media and mobile devices is negatively affecting people's ability to focus. In my opinion the only way to rule out that possibility would be to go several months without it, pretending like you live in 2005.

I know a psychiatrist who works at a university who said she has had many students tell her that they can't focus as well as they used to in high school. Since these students are in their late teens and early twenties, I doubt it is due to their age.