In my app, my uses enter their full date of birth. This is then converted to a UTC.

This means that for the same age, there could be potentially 365 values.

So let’s say that I need to create a compound index:

{
  dob_utc: 1,
  gender: 1,
  city: 1,
  ...
}

OR

{
  age: 1,
  gender: 1,
  city: 1,
  ...
}

Given that the field needs to be the leading field. Is it better to have age, fewer unique values, or dob_utc, 364 times more values?