this is a really hard one and tiny changes can really skew the opinion you will get.
The age of the parent, the age of the child in relation to other children in the same family, the child itself. All play factors.
the older generation of parents will probably say “in my day” and all was ok. Whereas a younger parent will be my hyper aware of danger because of how well we can access information now and it does scare you into thinking somehow the world is worse than it was 20,30,40 years ago. The same dangers were present we just didn’t hear about it as much. there are far more dangers on the internet now than before but as for playing outside is now probably safer as kids have trackers, there is more cctv etc. not saying it’s safe. Just safer technically.
I tend to find a parent who’s already had a child go through this independent stage is a little more relaxed and lets things happen a little sooner with subsequent children.
but I think the most important thing is the individual child. There are 9 year olds that are sensible, street smart, aware of dangers etc and there are 9 year olds barely wiping their own ass so to speak. Only you can judge your own child’s ability to be safe.
my opinion as someone who has children ranging from 4-16 is - 9 seems a little young to be out alone. In our school you cannot walk alone until you are in yr 5. I personally allowed my kids to start walking from year 6 and we only lived a 5 minute walk from the school. They started playing out in year 6 also and that is when I allowed them to walk to the shop alone.
we don’t know how far the school is, 2 minute walk or 10 minute bike. Big difference. Same with shops. Same with crime in the area. Village cul-de-sac is totally different to a big city.
What exactly is your friend worried about. There are a lot of different dangers - crossing the road, stranger danger, other kids. Does your child know what to do if he ever felt threatened in a situation. Does he have a phone to call you? Would he be easily led by other kids to go off and do something stupid or impulsive. Is the route to school safe as in low/no traffic, paths, safe crossings etc. is the route busy with people, shops, lots of other school children were strangers have less opportunity to see your child alone? Way too many variable factors to consider to really put in a worthy opinion. I think promoting independence and encouraging outdoor play is great. I also think 9 is probably on the younger side to start it. I would have given it another year. I also think all these dangers don’t go away when they hit 11,12,13. So it’s hard to say