I have a 6-year-old daughter. I don't let her play outside except right in our front yard. I work at a desk next to the front door, and have the front door open so I can literally see her play. I can't imagine letting her play outside of my site or the supervision of an adult or one of her teenage siblings until she's at least 8. If we had a fenced backyard, I'd let her play with a friend there, but we don't. I would let her play at a friend's house, if I knew the parents (not just met them before she went over, but really knew them.)
I had her when I was 33, so I'll be right at 50 when she graduates high school. I think you'll be fine on that front. For what its worth, I had SIX rowdy, naughty brothers and two sisters, both of whom were pregnant teenagers. I was petrified when I got pregnant (I'm the oldest, so they were in the midst of all this when I got pregnant with my oldest daughter.) But here's the thing. Both of my teenagers, while not perfect of course, have grown up responsible, drug-free and healthy.
I think the most important thing is to be aware of your kids. It sounds like you already are. Know where they are and what they're doing. Start talking to them young about things like drugs and alcohol. Be honest and tell them why you want them to avoid these things, not just that they should. One of my brothers is a drug addict and my dad is an alcoholic, and I've always been candid with my kids about their familial tendency toward addiction and why it's super important for them not to get started.
Being a parent is scary. But, the fact that you're asking these questions is a good sign that you're up for the task.
I think you get over being worried they are up to no good by learning to trust them. You don't have the benefit of having had 7 years with your oldest foster child, but by the time she's a teenager, you will have. You'll know you've taught her well.
Do you have someone--a case worker or maybe a friend who is a foster parent you can talk to? Do you belong to any foster parent groups online? I'm sure you aren't the only one with these fears.