I was 5ft7 and 7st naturally in my teens/young adulthood.
I was slim but I had no weight related health issues and I was a size 12 (never get the everyone skinny must be 'size 0' comments, size at low weight is based on bone frame not weight).
I had always been like that (was a very tall skinny child). I did not diet or do anything to achieve it and I was active and as well as I have ever been.
I only ever gain weight when pregnant, put on a stone per pregnancy and now at peak 'BMI' and 'healthy weight' ironically have far more weight and food related issues now.
I will likely never be 7 stone again (trying to diet down to that would be ill advised) but I was that size originally completely naturally and no 'worry' was needed.
If the weight is being lost for unknown reasons, an illness or extreme dieting/lifestyle change then worry, but its not a worry if someone is always just naturally 'skinny'.