When is a person a mature adult?

In recent years, we have distorted the concept of age in ways that simply do not make sense. For example, at the age of 18, one has enough agency to vote, but not rent a car or consume alcohol. A person can enlist in the military but cannot purchase a firearm legally. At what point is a person a fully mature adult? Is it an age, state of mind, or a singular act that signifies maturity? JC and B-Mic Roc OD.