I was really comfortable with my parents rules growing up: no dating until 16, and only group dating until 18. It kept me out of trouble, and we had discussions about any deviations from the rules that I wanted to try out. They listened to me and sometimes said no, and sometimes said yes and I was okay with that because I knew that they would take things on a case by case basis. Once I was 18, they would let me know what they thought was wise but leave it up to me because I was an "adult". I never had a curfew, but the car did... 
As far as maturity levels go, it really does depend on the person. I dated a guy about 5 yrs younger than me when I was 24 because he was far more mature than many of my peers because of some life experiences he had had to go through.
However, there ARE a few things that are pretty common when it comes to development--I don't recall specifics, but when I took my psychology class we delved briefly into the area of development of the brain and ability to process different things. Technically, we're not all fully developed until about 25 yrs old on average. So there may be a huge difference between a 16 yr old and a 19 yr old for that reason alone. 17 and 21 may be totally fine, I would just make sure I knew the guy really well and that they both spent a good amount of time around us and I would ask them to avoid being completely alone as I intend to teach abstinence until marriage in my house (It's never enough to just say "don't do it", you have to give them the tools to accomplish that, ya know?)

As far as maturity levels go, it really does depend on the person. I dated a guy about 5 yrs younger than me when I was 24 because he was far more mature than many of my peers because of some life experiences he had had to go through.
However, there ARE a few things that are pretty common when it comes to development--I don't recall specifics, but when I took my psychology class we delved briefly into the area of development of the brain and ability to process different things. Technically, we're not all fully developed until about 25 yrs old on average. So there may be a huge difference between a 16 yr old and a 19 yr old for that reason alone. 17 and 21 may be totally fine, I would just make sure I knew the guy really well and that they both spent a good amount of time around us and I would ask them to avoid being completely alone as I intend to teach abstinence until marriage in my house (It's never enough to just say "don't do it", you have to give them the tools to accomplish that, ya know?)







He had never kissed another girl before me. But he is one in a million and a truly wonderful man, so i think it definitely depends on the situation and I wouldn't put an age on it. 4 years isn't that much of a difference.
And, I also know, that in ANY situation not EVERYONE fits into the same category.
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