OP, this is one of those situations where there is a WIDE range of what people might do, and consider acceptable!!! So, it is kind of up to you and what you know what would be best for your kid and your family.
Just my take... Parent of a teenaged boy.
He has not done too much dating.
But OMG!!!! The really serious and heavy drama that he hears from some aquaintances!!!!
Anyhow, for now, I think he knows his limits, and is still deciding that he wants to proceed slowly. (I must be blessed!!!!)
Here is how it would work here...
A 15 year old dating... like one-on-one dating... wouldn't happen.
My thoughts are that once the teen is 16, has a drivers license, and has shown some level (no matter how small!!!

) of common sense or maturity. They hey, their social life is more of their own, and dating would be fair game. No sense trying to control what one can't control.
A 15 year old girl, and an 18 year old boy...
I also probably would not be very comfortable with that.
If May comes along, the kid has a full drivers license, has shown ability to look out for their best interests... and the actual number age-years in the age difference were two years, then I think it pays to be a stong and sensible parent, and the kid is going to find a way to do what they want to do... And just be ready to step in if it does become necessary.
About curfew...
Days start early around here.
Very early!
For both my husband and my son.
It is lights out around here by 10:00 during the week.
No matter what.
No way would I have a 15 year old child traipsing in and out and coming and going any later than that. Even earlier, unless there is an actual reason (event) that might make the later hour necessary.
So, hey, that is my .000002 cents, for what it's worth!