Plato’s Parmenides, written 370 B.C.E.: First Hypothesis, Second Deduction (154b-d) has Parmenides questioning Aristotle on the form of time:
b) If one thing is in fact older than another, it would be impossible for it to become older still by an amount greater than its original difference in age; and again, what is younger cannot become still younger. For adding equals to unequals, in time or anything else whatever, always makes the difference equal in the amount by which the unequals originally differed.
Of course.
c) So what is would never be becoming older or younger than the unity which is, since the difference in age is ever equal: it is and has become older, and the other younger, but is not becoming so.
True.
So unity, since it is, is never becoming older or younger than the other things which are.
No.
Consider then whether it is becoming older and younger in this way.
In what way?
Insofar as unity appeared older than the others and the others older than unity.
Well?
When unity is older than the others, it has come to be, I take it, for a time greater than the others.
Yes.
d) Consider then again: if we add equal time to greater and less time, will the greater differ from the less by an equal (proportional) part or a smaller one?
By a smaller one.
So whatever difference in age there was to begin with between unity and the others will not continue into the future, but since unity takes time equal to the others it will always differ from them in age less than before. Agreed?
Yes.
In modern notation: as two people age together, the difference in their ages will be a constant, but the ratio of their ages will tend towards $1:1$. So letting $a$ and $b$ be their ages with $a>b$, and $e$ the equal time added to both, then
$$e:e<a:b \quad \text{and} \quad e:e<(a+e):(b+e)<a:b$$