How old to walk home alone?

[crappyday]

DS has started to occasionally walk home from school on his own. I meet him at school (also picking up DD) and then he chooses to walk.
Country lanes, easily wide enough for car to pass him. No crossing roads. It's about a mile & it's a quiet rural area.
He's 9.
A few other parents think I'm irresponsible to let him. I think it's fine.
How old we're your DC when they started walking home / being out alone?

Ds3 (8) sometimes walks home with his friends round the road way whilst the dog and I cut across the fields and I'm going to stop collecting him after Easter but may linger in the fields with the dog until the summer. I'm one of the few that still collect from his year. Ds1 and Ds2 walked home alone at this age but he's P3rdB with a huge gap and an elderly mother. However it's only a 10 minute walk and just one small road to cross. I wouldn't be happy with country lanes but we're not that used to them.

I started letting mine walk to and from school on their own from Year 6.
So age 10 - 11.

I didnt start letting them go into town, to friends houses till end of year 7. 12 coming up 13.

DS walked home from school mid way through Yr4 (it's common where we are). He went to middle school from Yr5 & I wanted him to be confident. Busy town, lots of roads etc.