I will be the odd one out here and say that I partly oppose this.
Before I say anything else, I would just like to clarify that if sexual activity (or attempts of such) takes place between a minor and an adult here on MLP Forums, action should obviously be taken (we would of course take action even if it's a legal matter since mlpf is PG-13, but dealing with this would be a whole different story). Even if the underage person states that he/she is of legal age, this cannot be used as an excuse as the set age should be viewed as accurate unless the person in question can provide proof of their real age. "Minor" in this case would be decided by the laws of the country which hosts the site.
That aside, dating a minor is perfectly legal. Unless MLPF prohibits relationship talk ("I love you", "I want to hug you", "Be mine"), differently aged couples should be left alone after checking if their relationship is consensual. Staff should make no difference between the relationship of two 20 year olds and the relationship between a 15 & 30 year old, regardless of their personal feelings on the matter. Your disgust is irrelevant if there is no sign of illegal activity.
Furthermore, assuming that erotic role-play or similar has taken place outside of MLPF without evidence is wrong. Most likely it is accurate, but by asking the users in question through PM whether their relationship is OK on both ends, you have already done all you should unless they admit to doing something sexual themselves. If these people lie they probably do so because they both want it (and even if the young person do not, you can't prove this if they say otherwise), and it is none of your business. You can't accuse them for something you can't prove and it's not your place to investigate. Making an accusation based on false assumptions is even worse than ignoring the certain. Hunting down couples who happily engage in illegal sexual activities together is a system I want no part of. Partly because I think it's not your cause to examine, partly because it doesn't happen on your platform and is therefore not relevant unless there is proof (even less so if it happens between two people in a country where it is legal since they don't show it on the site itself), and (very controversial, I know, and I don't care) partly because I do not agree with the laws surrounding underage sex. (If anyone for some reason is interested to know where I stand, open spoiler)
I recall reading a post about having to prove ourselves as a caring community by actively searching for "predators", but what's the point in that if you don't have any evidence? Why the need to search when you can be optimistic and assume everything about the relationship is legal? Might as well open up staff applications for an investigation position to filter out all drug-users on the site as well if that's how you're gonna be like. You're not the police and unless you have a reason to accuse someone for being a sexual predator, don't act like it.