Silicone Implants on an 18 Year Old?

You are right, the FDA had approved silicone gel implants for those 22 and older. The decision was arbitrary and is some ways seems unfair. On the other side of things, since gels were removed from the market, we used saline implants for 15 years with considerable success and we still use many of them today. The cost is lower, the incision smaller, the leak rates similar or lower, the late contracture rates lower, and ease of replacement is remarkable, the softness very nice and better than very cohesive gel. Finally, as implants last perhaps 15+ years, you can get gels (if you want them) at the age of thirty something if the saline has worn out, and yes, no MRI's. Not really so bad.

When silicone implants returned to the market the FDA approved those implants only for women over 22 years of age. this is a guideline there may be some surgeons who offered these implants on an "off label" basis. 

However the large majority of plastic surgeons will follow  the rules sit down by the FDA.

Thank you for your question and best wishes