Alright. All the definitions I've seen in this thread are terrible. To make this rigorous, you cannot define early-, mid-, and late- as a function of age unless you want to form unequal partitions. I propose the following construction.
Definition 1: The discrete
age of a human \(H\) is equal to the floor mod of the time, \(T\), in years, since their birth. That is \(\text{age}(H)=\left\lfloor T\right\rfloor\).
(This is here only to clarify what I mean by age).
Definition 2: We say that a human \(H\) is in their
twenties if and only if \[20\leq T<30\quad \big(\!\iff 20\leq\text{age}(H)\leq 29\big).\]
Definition 3: The twenties can be broken into three equal partitions \(t_1\), \(t_2\), and \(t_3\), which we name
early-,
mid-, and
late- respectively such that \[t_1=[20,\,70/3),\quad t_2=[70/3,\,80/3),\quad t_3=[80/3,\,30).\]
I'm sorry, I had to go full maths nerd on this