There is no rule against including HS employment or similar experiences on your medical school application. If you continue on the job through after HS, you can certainly list the start date in 2018 even if you are still in HS at that point.
Typically, a college students and applicants who have been out of college awhile will have so many activities to list in the experience section that they will fill the 15 slots with more recent things rather than going back to the activities in the more remote past. However, if an employment or honor or activity is very important to you and you wish to list it, feel free to do so.