I think you should just maintain healthy eating & exercise habits and let your body be however it's going to be. You should also consult with your physician to make sure everything's OK (i.e. your BMI is within normal limits, maybe check out your thyroid if you're concerned, your diet/exercise routine is suitable, your own body image perception is healthy, etc.). As for exercise, it's always good to have a mix of different types of workouts - cardio, strength, flexibility. But yoga & pilates are awesome for exercises that give you tone and a nice lean look that would probably look awesome on your skinny-minnie frame. Both yoga & pilates are good for flexibility and certain types can give you a decent cardio & strength workout (but probably best to do other activities as well, IMO). In any case, I'm sure you're a beautiful person, bones & all!!
btw, metabolism does slow down as you age. That's a fact, but I'm seeing it myself... I always hovered around the 88-lb mark when I was in high school & univ. but have gained 10lbs in the past year!! And no real change in my eating habits (actually, ate probably a little *more* junk food back in school). That's life, I guess. Anyway, sorry for the lengthy post... HTH!