In terms of healthy relationships - no, this isn't okay in the slightest. An adult should not be dating a child, there is a lot of difference in terms of neurology and maturity between the ages of 13 and 18 years old so it would be an unbalanced and thus unhealthy relationship that could leave the girl at serious risk of abuse. An 18 year old wanting to date a 13 year old points to a questionable person.
In terms of legality it depends on the state or country, dating would perhaps be legal but any sort of sexual activity would be criminal and count as sex with a minor.
There are no laws regarding dating. There are laws regarding sexual contact, and this would be a very bad idea.
no
yes
Yes it is, but only if he is within a year or so of the younger party.
Absolutely not.
No. Until he turns 18, and but with the minors parents permission it is okay.
no it is not
only if your a girl.
Yes, if it's ok with your parents. But, guys in their 20's usually want to have sex.
Yes, it can happen if your okay with having a boyfriend/husband be older than you.
No.
no...
That's not a relationship that's a pen pal that comes to visit
Yes.