Which technique is best?

Hi, i'm final year undergrad student and have been stuck for quite a while now. I have been provided with data on the girth measurements from 12 points on the body from 100 people and from that I need to come up with a measure of girth as a whole. I really don't know what technique to use. Somebody suggested Principal Component Analysis but I don't see how I could use the results I obtain from that for the second part of the question. The second part of the question is to take that measure of girth and apply it to different age groups to see if it changes significantly.

Any help and advice would be greatly appreciated.