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

I'm absolutely against such relationships. Tolerable age difference imo would be a year max. They are still learning about love, relationships, etc. They're still exploring that territory. They're gonna expect everything to be hunky dory like in some of those movies etc. but no. They're gonna feel disappointed due to that which will distract them from their studies,etc. The mental age gap is gonna be too much. And the list continues......

Well, the most honest answer would be: Because it's illegal. And that's pretty much it. Sure, back in the Medieval times people had sex and relationships with teens above 13 (Hebephilia and Ephebophilia) and even today some countries have the age of consent set at 14. But for the most part, modern society does not tolerate such relationships anymore. And that's why most countries worldwide have an age of consent of 16. The reasoning is that, at 16, the person has the ability to consent and make decisions. Any psychiatrist will tell you this. Also, biologically, both sexes are mature and able to conceive at 16. So there you have it.

I would not date anyone under 18 for sure. It is too much of a fire hazard when it comes to passion and hormones. That said, age difference is not something that is hard to overcome so long as people are able to listen first before rushing to make a connection. Understanding what each other is trying to achieve in the relationship is what decides the longevity. More to the point, my father and mother are 10 years apart with my mom being younger. They have been together 28 years. At the same time my brother married up by 9 years to my sister-in-law. Their marriage has lasted 10 years so far. In conclusion, don't dip into the 15 and 16-year-olds on principle of having a miserable time and being a creep. However once you are 18 the age gap comes down to whether the people want to be together and have enough in common. I don't see adults who are apart even by 20 years as bad as long as they stay committed to each other. Love isn't sex, it is peace of mind and security.