OK Sally that is a bit of a difficult one to answer, the problem being that everyone is built different.
Muscle weighs more per square inch than fat and for someone who is 5'2 and weighs 7.5 stone and is lean can look very different to another person who is 5'2 with less muscle who will look a bit more on the dumpy side.
My wife is 5'5 and just under 8 stone. Goes to gym 4xper week.
Her best friend is 5'5 and 14 stone. No gym per week.
and they are both the same age so a hard one to answer!!
So if you feel you are under weight and not happy you could always consume a few more calories through out the day at a steady rate to put on a bit of weight.
Do you have breakfast!, Have some fruit as a snack!, Glass of milk before bed! or you could try a weight gain shake which make consuming more calories a bit more easier than eating them. Most have a good selection of vitamins and minerals in as well, just take it slowly at first and monitor your weight once a week at the same time, say first thing in the morning on a Friday.
Hope this helps.