It's a bit fuzzy, but more or less each gen gives birth to the gen after the next one. Boomers are the parents of millenials, not the grandparents. Gen X are parents of Zoomers. Millenials are the parents of Alphas. Etc.
Right now Millenials are in their 30s and 40s, Zoomers are in their 20s and late teens, and Alphas are young kids. Most Zoomers are not having kids yet.