The age of the son being addressed by his mother is never stated in this poem. However, we have clues that help us determine how old he is. For example, although she calls him "boy," the way she speaks to him indicates he is a young adult. He does appear to be at a crossroads, ready to embark on his adult life journey and fearful about it.
The mother speaks to him with a frankness with which one would not speak to a child. A child is not yet ready to face life and does better if protected from some of its harshness.
However, at this juncture, the mother wants her young adult son, who is black, to be prepared for what he will face, which is a harsh world. She tells him that experience has taught her that life is hard. She says several times that "life for me ain't been no crystal stair." Instead she uses the extended metaphor of a staircase to describe a life where sometimes it is dark, and sometimes the floor has holes, or tacks, or splinters. He should, in other words, prepare himself to be hurt and meet with obstacles.
Nevertheless, the mother uses herself as a positive role model for her son, saying that no matter how hard life has gotten she has never given up and he shouldn't either:
So boy, don't you turn back.
Don't you set down on the steps
'Cause you finds it's kinder hard.