Different countries have different ages. There were some states here in the US that allowed kids under 18 to get married - they might still do that in some places, I'm not sure.
Love shouldn't be restricted, yes, but there's a problem in that you can't prove that something is really love. The reason for prohibiting relationships between minors and adults is that in general minors are not experienced enough to protect themselves. Some people as we've seen are not even aware that this might be illegal.
In other words, I would propose that a minor doesn't have the level of judgement necessary to agree to such a relationship. I sure thought differently when I was one, but there's a big difference in even being one year older. Any adult in a relationship with a minor is probably taking advantage of them, or has some psychological problems.
I might even go so far as to say the limit of 18 years in the US is probably too low. Well, in some respects anyway. There's a big difference between Jr. High and High school, and an even bigger difference between High school and College. I don't know that there would need to be laws per se (aside from College kids, being over 18, are subject to 'jailbait' laws), but definitely such a situation should be monitored by the parents.
Furthermore the things like 40-year-olds hitting on and dating college-age kids is totally wrong. That in my opinion should be subject to some laws (college kids, while more mature, still aren't on the same level as professionals).