Although your growth plates (areas at the end of each bone) will have fully formed, the density of your bones continues to develop throughout your life-time.
When your bones are regularly exerted to the kind of force that running places them under, for example, the density of the bones in your legs increase as they’re forced to adapt to the downward pressure that passes through them with each footfall.
Similarly, the increase in muscle mass, achieved through weight training, has an effect upon bone density, hence the reason resistance training is recommended as means towards reducing the risk of osteoporosis in later life.
As I hope the above demonstrates, your level of weight bearing exercise, throughout your lifetime, ultimately continues to affect the weight of your skeletal system.
Concerning your question about body fat, that certainly is something that can be reduced, provided you consume fewer calories than you’re expending, in addition to performing cardiovascular exercise as intensely as you’re able to manage.
The more intense your cardio exercise is, the more fat you’ll burn in the process as it’s utilised to fuel during your endeavour, hence why HIIT has risen to such prominence in recent years. Additionally, the more intense your level of cardio, the greater the after burn effect (energy deficit), meaning that the carbohydrate (yes, carbohydrate) you consume in the hours afterwards is broken down and used to replenish depleted glycogen reserves and less likely to be stored as fat.
If you want to lose weight quickly through intense cardio, you need to consume carbohydrate following the activity, since protein and fat won’t replenish depleted glycogen in the exercised muscles.
I’m not necessarily saying that intense cardio will reduce your level of body fat to that of your teenage years, but the activity will significantly reduce the amount of body fat that you may currently possess.