How old does a royal poinciana tree have to be to flower?

A height of 35-40 feet [10.668 - 12.192 meters] and a width of 40-60 feet [12.192 - 18.288 meters] is what a Royal Poinciana [Delonix regia] matures to. Woody plants such as shrubs and trees never stop growing. It's just that the mature size is the point at which the processes of growth are stronger than the breakdown processes of decay. After that point, any step forward in growth is cancelled out by the ever more assertive steps backward into old age, decline and death.