Why is age difference bad in adolescent relationships? (ex. 15 and 20, or 19 and 14)

Mainly it is a legal/moral issue. Different states have different opinions on what age is legal, in Washington it is 16, but otherwise it is universally accepted that 18 is the borderline. There are exceptions in arranged marriages and engagements, but that is far too complex a topic for me to go into.

In my opinion, it's iffy to talk about the right age gap before the age of 18. Like, a 17 year old with an 18 year old usually is not frowned upon. During adolescence, the mind and body are more or less rapidly maturing, so I would normally say a 14 or 15 year old is much different than an 18 year old. They're in two separate states of mind of maturity. Post-18, it's fair game for anyone legally, but the average age difference is three years for couples over the age of 18 in the United States. I personally have no qualms with, say, a 21 year old with someone aged 30+ as I was.

As for the forum topic, age difference is bad in adolescent relationships because, I think, most people feel as if one's maturity determines whether someone is willing to exploit the other and that age determines maturity. There is also the difference in expectations, needs, wants, life experience, and whatnot. When we think of adolescent relationships, sexuality is often inextricable from the concept and as far the West is generally concerned, sexual innocence is important. We also have to think of who frames the concept of age difference in adolescent relationships as bad. For, say, soccer moms, they may be concerned their daughter is going to be exploited by someone who is much older, by someone who has sexual urges. Then also, a 15 year old is going through high school normally. A 20 year old is either in college or working a job. They're not going to be able to relate as much as they would with someone their own age. Apart from differences in life experience, we could debate as to why any of this is morally bad, and if we take a cultural relativist's standpoint, it's bad because society says it's bad using, possibly, some of the reasons people have mentioned in this thread already.

In anything under 18, I'd say anything north or south of 2 years or 2 grades where you currently are is reasonable. Once you get north of 18, then the divide by 2 + 7 rule is a pretty reasonable baseline to work with.

Hypothetically, then, a 17 year old and a four year old could "date" with that logic of "[b]ut minors should only date minors".