Certified GKH Gardening Expert
They usually flower fairly young. If it isn't flowering, then there may be an environmental factor affecting blooms.
They need to be planted where they can receive full sun. The more sun the better the flowering. If the tree is shaded, it may not produce well.
Soil nutrients are important, too! If the area contains too much nitrogen and not enough phosphorus and potassium, then it may not allow for proper flowering. The same can be said for soil that is too alkaline, or too acidic. The soil needs to be pretty neutral at 7 or near it.
This article will help you to take a soil test: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
This article will help you to care for these trees: https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm