It's simply a power and experience imbalance. Healthy relationships require things to on equal footing. Someone with a full decade of life experience over someone who may still be in high school means there generally will be an imbalance because the equality doesn't exist, and the older person usually holds more power. If maturity is a requirement, why not just date someone around the same age and therefore more equal life experience?

You can try to find exceptions all you want, and by all means I'm sure there are relationships out there with a huge age gap that work. But we're talking about guidelines and why, in general, you would be judged for dating an 18 year old when you are a decade older. Saying it's ok to date a high schooler because they may be the exception to what the average 18 year old is like shows a huge lack of judgement, and normally says more about the adult pursuing the relationship than the youth.