Q.How old will my Mimosa tree need to be to start blooming? I have given it no special care, and grew it from a tiny sapling. The
tallest branch is about 8′ tall. It is at least 3 yrs old. I live in zone 5 in Muncie, Indiana. I got the start from my daughter’s tree in Indianapolis, Indiana. The house has been sold and the original tree that was beautiful every Fourth of July has been taken down. We had to look it up to find out what kind of tree it was. I will put up with the messiness for the beauty. Hope to get her a start from mine for her new house!

Certified GKH Gardening Expert
It can take a few years before mimosa/silktree starts blooming, but at 8 feet and 3 years you may not be far off now.
https://www.gardeningknowhow.com/ornamental/trees/mimosa/growing-silk-trees.htm