I appreciate this answer, because it at least tries to reason from first principles. You can't, imo, have this conversation without actually defining what we consider to be the problem.

I think the key concern is that age -- particularly during teenage years -- typically correlates with a power imbalance. And the concern is that the younger person could be exploited and/or suffer harm. However we need to remember:

1. It's possible for relationships to have a power imbalance and no one is harmed or looks back with regret.
2. It's possible for relationships between people of the same age to be very harmful/regretable.

So the questions I have are: how correlated is a specific age gap with severe harm? And what would we advise in this situation?

I think that a 16 year-old probably has around a 50% of getting badly hurt in a relationship with another 16 year-old, and probably a ~65% chance with a 19 year-old. Because a 19 year-old can probably manipulate a 16 year-old better than their peer, but they're also presumably a bit more experienced and mature, which can be a good thing.

I'm making these predictions presuming that they're sexually active, btw. Which I think is probable. But if they're not, I think that the risks go down to around 10% chance in both cases. This is just my gut impression. So I'd just advise any 16 year-old in a relationship with a 19 year-old to move VERY slowly physically, and talk frequently to an older friend or sibling. And if your partner wants to do anything you're uncomfortable talking about with your older friend or sibling, that's a sign you shouldn't do it.

If you follow that rule, I think 16 and 19 is no big deal. Because I really want to emphasize: a lot of the risk already exists when a 16 year-old dates someone their own age.