In any case, my original sentiments still stand: it's not impossible for a 15-year-old and a 19-year-old to find success in a relationship with each other, and I won't condemn anyone for trying, but the odds of them actually doing so are very, very slim. In the teen years, such an age difference would usually mean too much of a disparity in terms of maturity, expectations, and life experience. In other words, a recipe for failure.
I've known people of those ages who were in relationships, and while the relationships haven't worked out, they lasted a good long time, especially given the ages of the participants.
I try not to judge, within reason - if I knew a 19-year-old (or older) person who was dating someone that much younger than they were and I could see that it wasn't a healthy relationship, I would say something about it. But if all was going smoothly, I wouldn't see a reason to get involved.
Oh, absolutely -- if there was something obviously unhealthy about the relationship, I would probably try to talk one or both of the participants out of it too. I'd try to do so in as least meddlesome a manner as possible, however.

I might have been overstating it to say "recipe for disaster". As I stated upthread, there are always exceptions to every general rule.