I grew up an only child with lots of time for reading. Then I grew up and married a man who was not a reader. While I made sure to continue to make time to read, because I was busy setting up my own house and discovering new hobbies and interests, I was reading fewer books.
However, I'm still getting older
and I'm definitely reading more fiction now, compared to when I married and set up my own house. As my kids get older and we buy them books, I find that I'm reading even more just to keep up with not only my intestests but also with their interests so that I can get ideas for what books to get them next.
I think it is less that reading declines as we get older and more that it is cyclical, depending on where we are in our lives and what is currently most important to us at that time.