Every airline has different rules re age a child can fly alone. 17 should not be a problem. My daughter was flying Faro to Gatwick from 13 but only BA would take her. She did not fly as unaccompanied minor after the first time as she said it was too embarrassing!
I wouldn’t call a 17 year old a child in these circumstances. Surely a mentally and physically able person of that age could find their way to, from and around an airport without a chaperone?
Good God, I was living on my own in London, in my own flat, working and making a decent salary at 17. I also, for work, was travelling all around the UK, taking care of my own accomodation, travel etc. Also I recall, arranging an exhibition in the US and then flying there, all alone, to work it. Or maybe that was when I was 18............
I think that a lot of airlines have done away with their UM service, at least the ones our grandchild used. I recall the sons of friends, once the eldest was 12 he not only could travel unaccompanied but he could also be the official guardian of his 10 years old brother!