Our front yards are very active because of the neighborhood. Yes, it is a major thoroughfare but once the children were 8, 9, 10, they had learned not to go into the street not even to go past a certain place in the yard on the way towards the street.
Of course these were children that were all pretty intelligent and mature. We had NEVER, since they were toddlers ventured towards the street. They played in groups and we were checking on them quite a bit.
We each had known our children since they were born and as a group of parents, we had agreed on the same rules, expectations and respect from each child.
They knew if we saw them breaking any of the rules, they would not be allowed to play out there. And we would speak with the parents if their child broke rules.
We had a fun "Air Pogo" and a "Twizzler" on one of our front trees, because it was the only tree that could hold them high enough.
At that time we did not have sidewalks.. So we had each made a visual marker in our front yards that the children knew not to cross.
I think you can decide based on your child and their maturity and decision making. Where you live. And how often you are able to check on them and how you can view them.. Front door open? All glass front door? Large front windows?
I know there are children waiting at the bus stop in the dark early in the mornings. They are in elementary school. Then they are dropped off in neighborhoods to walk home by themselves. IF they can handle this, I think they can play in their front yards.. again based on their personality, behaviors and the neighborhood.