Marriage age

Peace, Your Honor,
I would like to ask you a few questions that concern me:
1. What is the most appropriate/right age for marriage? Is there one?
Tractate Avot states that the marriageable age for men is between 18 and 20:
"Eighteen years old for the wedding, and at the latest, by the age of twenty."
This is in order to prevent thoughts of transgression, and later it is stated that this can be postponed for the purpose of studying Torah.
In addition to this, according to the Rishonim (I have not found a source for this) it is written that in any case he will not be late after the age of 22 or 24.
2. Is there any mention in the Torah that the correct/recommended age for marriage is 18? Or is there a mention for a different age?
3. How can one determine such an age range, which is most recommended for wedding for the entire population? For we know that the emotional maturity of each person is different and varies for thousands of reasons.
4. What are the offenses referred to when it is written " contemplation of an offense" ? And are there other serious offenses that are exposed to when in a relationship without marriage?
5. Why is the argument of the mitzvah of "prove and multiply" brought up? After all, if the couple gets married, even if it is at the age of 30, they will still be able to fulfill the mitzvah.
6. I would also like to understand the origin of the prohibition on having sex without marriage between spouses and why it exists.
Thanks in advance.