If I had to give a scale I would say (on my opinion):
0-13, Child
13-18, Teenager
18-25, Young adult
25-40, Adult
40-60, Middle age
60-80, Old
80-95, Senior
95+, Idk man at that point does it even matter anymore?
So I would say that "Young" would be from 0-25 from a technical standpoint, but...