Young pilots in a future space war short story

I read this short story in the late 1970s. An anthology from a UK library…no memories of the other stories.

A space pilot is sitting in a compartment on a train, another passenger boards and sits opposite, he notices the pilots pinned up empty sleeve and starts murmuring about "we owe you a lot"

The pilot tunes him out and lets him chatter away while he reminisces about his time in the Space Fleet. As the train waits to leave the station he hears a prolonged hypersonic boom from the upper atmosphere, he identifies it as a ‘something something’ interceptor at maximum speed.

The passenger leaves a couple of stations later and soon it’s the pilots turn to disembark.
He starts walking down a country lane to the family ranch and thinking about his birthday tomorrow
"Man, thirteen is sure gonna feel old"