Q.Do mimosa trees lose fragrance as they get old?
My mimosa trees used to have wonderful, strong fragrance. My young granddaughter would lie down under the trees and say that she wished we could bottle that fragrance. In the past few years I have noticed that fragrance is entirely gone. Hummingbirds who used to flock to the trees ignore them. They do bloom profusely.

Certified GKH Gardening Expert
Yes, this can be normal. These short lived trees only live for one or two decades before dwindling out. Enjoy what it does produce until its demise.
It's one of those trees that you hate it or love it. Many people have fond memories of the tree during their childhood. Unfortunately, it produces a bounty of seeds that come up anywhere and everywhere. And the tree can suddenly die on you with no apparent reason. Plus it is pest prone. Most tree professionals advise against planting it. So if yours is on the way out, you might consider something else as its replacement.