Is legal when you're a minor?

I kow that a lot of teens have and stuff, but is it legal for toddlers to? (obviously this is hypothetical) is there a cutoff or something? And I know a 30 yr old can't have with a 17 year old but can a 18 or 19 year old? Just cause I don't really know much about the laws and stuff. And what about kissing/dating? Can minors and adults date?