My kids take the train to school.
So they started aged 11.
They go on a local line and there are loads of other school kids on the train. Train journy 30 minutes.
It was a huge thing for them when they started doing it in year 7. Veyr daunting to be waved off and have to get off in the right place. With each of them there was an older student looking out for them. But with DC2 and DC3, DC2 decided to go into town on about day 3 and left DC3 to find their own way which was particularly unhelpful.
They did do it once at the end of year 6 for their transition day, so some of the kids may have been only 10 at the time.
The thing I would say though is that when ds started doing it, 10 years ago the trains were so much more reliable. Now trains are regularly cancelled, terminated, stop at one station for ages due to an incident further up the line and so on. They are also regularly late.
My kids are really chill about it and take it in their stride, BUT there are 60 school kids on that train, and they all sort of know each other, and look out for each other (even when they pretend they don't) The staff at the station at each end know them all too, and are usually really kind to a lost looking younger kid.
A school run train is very different fomr a random train. I think you are about 2 years too early. I would let them once they are both secondary age, because their degree of indepence shoots up enormously once they start secondary.