I'm confused once again. What would be the difference between the following two than?
1) By the time you hit your late twenties you've dated a few people.
OR
2) By the time you hit your late twenties you will have dated a few people.
The same difference as there normally is between utterances with 'will' for certainty and those without.
One of the things that appears to cause you confusion is that you often introduce completely separate issues.
Which one is really puzzling you??
1a.
By the time you hit your late twenties, you
have dated quite a few people. (general time, fact
1b.
By the time you hit your late twenties, you
will have dated ... . (general time, certainty)
2a. By the time
you hit (present simple) your late twenties, ... .
2b. By the time
you've hit (present perfect) your late twenties, ...
3a. By the time you
hit (present simple) your late twenties, you will have dated quite a few people. (future time)
3b. By the time you
hit (past simple) your late twenties, you had dated quite a few people. (past time)
Before you ask how we tell the tense of the verb in 3a and 3b, or the time of utterances 1b and 3a, the answer is 'context'.