I promise not to let this degenerate into one of my whines about step kids. Just humor me.
Let's have a hypothetical...18 yo young adult children. Living in your home in anticipation of going off to college next fall. You provide room, board, etc. You will be helping pay for their college education so long as they are performing reasonably well. They are high school seniors, reasonably responsible, have part time jobs that they save and use for their discretionary spending, pay for their gas, insurance, etc.
At 18 and living in your home I think you certainly have a say as to reasonable hours to be in, quiet enjoyment of the house by all living there, etc.
Not really as long as they are not disturbing anyone else regularly. Being annoyed about it does not count.
But what about their comings and goings?
No
Should they be allowed to have spend the night company of the opposite sex in your/their home?
Your house, your morals, your rules on that one regardless of age.
Should they be allowed to spend the night with their bf/gf's away from your home at the bf/gf's house?
Yes, again you don't have to like it but at 18 they are adults.
Should they be allowed to go away for weeekends with bf/gf's so long as they tell you where they are going to be?
Yes, but telling you is the courteous thing to do, not mandatory.
If they are making such "adult" decisions unilaterally and not actually asking for your permission but rather merely informing you of their whereabouts/actions so you will know what is going on, is it time for them to assume adult responsibilities such as gettting there own apartments/pay rent/pay for their own food, etc?
They should be paying rent if they are working, car insurance as well.
Would your answer change depending on the gender of the child?
No
Should they be allowed to come and go as they please so long as they keep you informed?
Yes, but informing you of comings and goings is again a courtesy.