They should be at a good age to start free ranging. My first flock was 12 weeks, the second group I've had were with an adult guinea who knew the ropes, so I let them free range at 8 weeks. If I hadn't had to raise them by hand and they had guinea mamas, they would have been free ranging even earlier.
Sometimes I've found they need help getting back in - if you think about it, their perspective is from the inside out and they need practice going from the outside back in. And don't be surprised if they don't go out the first day or so. They may need time to figure things out.
I'll free range mine in shorter intervals at first and make sure a light is on so they aren't afraid of going into the darker area from the bright sunlight or if you're still trying to get them in as dusk arrives. I try to get them in an hour before sunset so they aren't tempted to roost in a tree. I do train to a bell, so that helps and reward with millet once they are in. I'll stand outside the coop, just to keep them from going around and around.
Hope this helps! Good luck! It's always a nail biting experience at first and when they get it, you have a sigh of relief. FYI - I do encounter some problems when they start to pair off in the spring. The males are more interested in "getting the girl" than "getting in the coop". I usually have to pull out my arsenal of flashlights as they are still struggling to get in well past sunset.