Personally I think it should always be up to the kids (the idea that a child has limited rights under our US Constitution is ridiculous).
Legally that would depend on the country, state, and sometimes even the city.
This is no means a legal or up to date list but if has good information for all over the world:
http://www.ageofconsent.com/ageofconsent.htm
NO
No.
yes
It is not illegal for them to date, but is is illegal for them to have sex.
No, it is not illegal for a 15 year old boy to date an 18 year old girl in the state of Illinois. However, it is technically illegal for them to have any sexual contact.
they sure can!!!!
No.
anywhere?
NO
No.
no you can't date a girl that is 5 years old
Yes
no
no
No..but yes
kind of
yes