Do you think parents not letting their kids date until their 18 is unreasonable?

Parents are so strict and unapologetically keeping it that way. 16 is a good age to start dating. I wasn’t allowed to have a boyfriend until senior year. My dad told my siblings and I about what we needed to know to prepare us for the future. He said life isn’t supposed to be comfortable, it’s about challenges and that’s what he wants us to do.
Anyways my brother and sister in law were saying they aren’t letting their children date until their 18. They’re going to rebel on them. My nephew is 14 so I agree he’s too young.
My niece will be 12 later this year (too young) and my other niece is 8 (definitely way too young). My sister in law said she had her first boyfriend at 15 and shows in a relationship and that’s and also her grades got behind. She said that’s why she and my brother made that rule. No dating until age 18 and out of high school but we will teach them about being safe and the ups and downs.
I think they’re being unfair.
I admit I did listen to their conversations but didn’t say anything. Lucky they didn’t caught me. I’m so disturbed by their strict rules. I understand they don’t want them bringing babies home. I don’t blame them for that part.