For a person of small build, it might not be underweight.
In my 20's, I was 8 stone ( briefly) at 5'4. I still had a bit of a spare tyre , definitely not a flat stomach, and was never less than a size 12. A lot of my friends were lighter than me, but not skinny.
But standards were different- a 25" waist was not considered particularly small, ( Etam did a 28" waist skirt, which was for larger girls, size 16, a size not available in many shops.) Lots of shops had only 10 (22" waist) 12 (24) and 14 (26").
7 stone was a fairly unremarkable weight,certainly for a teenager.
Anyone familiar with vintage clothes will know they are very small .