Double Chin Even Though I'm Very Slim?

This is all too common. The good news is you are the perfect candidate for treatment. In my hands below are the best options, though there are many!
Option 1 : liposuction/sculpting. This is done in clinic under local with maybe some medication to make you feel calm. 1-2wks of post op swelling and brusing. 
Pros: quick results, minimal down time. 
Cons: a procedure, small scar at lipo cannula site

Option 2: Kybella injection: done in clinic, no real acute downtime though patients often describe a very "full sensation" with a heavy neck for up to 2 wks.
Pros: done in clinic, minimal down time
Cons: multiple treatments often required, results takes a few months to expose themselves 

I always discuss fat transfer along with liposuction. It can help sculpt the surrounding areas. Not a must but a very nice adjunct.
Good luck.
DrC

Yes, submental fat under the chin can be the result of many factors besides weight gain and genetics is at the top of the list. The good news is that there are treatment options for this issue that are appropriate at any age. CoolSculpting is a noninvasive procedure that uses controlled cooling to freeze fat cells. Since fat freezes at a higher temperature than other cells of the body, the fat is destroyed while surrounding skin and tissue remains untouched. Results take time to develop as the body naturally flushes away the treated cells. However, the final outcome will be long-lasting, up to 15 years in a recent study published by our group.

If you have a significant amount of submental fat, liposuction is another option. I perform tumescent liposuction using micro-cannulas, which provide superior precision and results while shortening recovery for most patients. The procedure can be performed using local anesthesia and recovery is typically just a few days. This procedure offers more dramatic improvement after a single treatment session and results are also long-lasting as long as you maintain your stable weight. I would schedule a consultation with a board-certified dermatologist with extensive experience in facial contouring procedures for a full assessment and treatment recommendations.

There are many people that have a double chin, even though they are thin. A double chin could be a genetic thing. liposuction or liposculpture can fix a double chin, and the recovery is very easy!

A double chin can commonly be a genetic trait, especially if you're thin.  Sometimes, it is accompanied by a small chin as well.  Treatment of a double chin can be performed with Coolsculpting, Kybella, or neck liposuction.  Coolsculpting is most popular because it's a nonsurgical option, has little downtime, and gives good results.  Neck liposuction is the most definitive and treats the entire double chin area.  It has about a weeks worth of bruising and swelling but really no pain but because of the downtime, people often opt for coolsculpting.  Kybella is performed less and less because it's a nonsurgical solution but has as much swelling as neck liposuction in many cases.  

Submental fullness is a very common complaint and concern. There are many options available on the market today from coolsculpt, kybella and other noninvasive. This can also be addressed more fully with liposuction, using a small incision under the chin. Liposuction is my preferred approach as it can be done under local anesthesia in the office. Recovery is some swelling and bruising for 1-2 weeks. Kybella and coolscupt also cause swelling for 1-2 weeks and often require multiple treatments. Sometimes a chin implant is also essential to improve the jaw line and facial contours. Have a consultation with a specialist that has all of the tools to offer you.  If you just see an injector you'll only learn about kybella.

I could better answer your question if there were pictures provided. However, there are several options to treat submental (chin) fat. Since you are young, your skin is likely to be of good quality which makes you a good candidate for Kybella and liposuction. If you have loose or thin skin, then these options may not be the best fit.Kybella is a simple in office treatment that involves injections of an acid that dissolves the fat cells in your chin area. I suggest doing 3-4 treatments 6 weeks apart for optimal results. The downtime associated with kybella is having little-moderate swelling of the chin area for 1-2 weeks. Liposuction involves a short amount of time spent in the OR, and the results are more dramatic and obtained quicker than doing a series of Kybella injections. There is a 2 week down time of limited activity levels, and minimal swelling in the area for 4-6 weeks. In good heath,Dr. Ellen Mahony.