I am an average like person, not under or over weight. I like staying active and my jaw bone is prominent
you might enjoy increasing your activity levels overall with the "side effect" of increasing your lean mass and reducing your fat stores generally (which should sculpt your under-chin area as a bonus). This is only a good option if there is an activity or sport you think you'll be doing a lot more than at present. Something like running is ideal because it can be quite addictive. Other sports are excellent too. Circuit training is great.
There are neck exercises (for boxers usually) that will tend to define your under-chin area mostly by increasing muscle strength there. These are hard work, and painful afterwards. Only do 5 minutes a day max, once a week is plenty, and be careful not to injure yourself.
i had this dilemma too and everyone said you have to lose weight overall to lose the chin. so just go on a diet, reduce your calories and exercise more.