I believe falling out of love has less to do with a person's age and more to do with the circumstances of the relationship and what is going on in an individual's life.
Sometimes the people we fall in love with changes overtime and sometimes it is us who change.It is not uncommon to hear someone say; "She/he is not the same person I fell in love with." When we change our circumstances change. If you gradually stop doing the things that caused someone to fall in love with you then it's bound to have a dramatic effect on your relationship.
In other instances our wants/desirable traits in a mate change. What was "ideal" for us at age 18 may not be what we want age 25.
At other times people develop different relationship goals. They may have had the same plan when they got together but after awhile their priorities changed. Let's say the plan was to have one person stay home to raise children, however this person gets a great career opportunity that they do not want to give up or they decide they don't want children for several years if at all.
Last but not least is the "midlife awaking" which is when one realizes they have more years behind them than ahead of them. Personally I don't believe it's a negative thing to become aware of just how much time one is likely to have left to pursue goals/dreams. During the 90s the phrase "bucket list" became very popular. All of us have some things we've always wanted to do whether it's to learn to play an instrument, learn a new language, travel someplace special, or own a particular thing. It's part of what makes us individuals. Life is a personal journey. Unfortunately sometimes the person one is with does not want any dramatic changes. They're happy with the way things are.
We're either growing together or we're growing apart.