Is minor heart pain after the first workout in a few months normal in 18 year old adolescent or does is it a cause of medical concern?

You mean chest pain?: I would say if it is truly "heart pain" then it is never "minor", and always cause for concern. However i assume you mean pain in left side of chest. In an 18 year old, this type of pain, especially after doing anything involving the chest or arm muscles, is most often musculoskeletal and not dangerous.

Not heart: 18 year olds don't get heart pain (okay, you can never say never); but unless that person is diabetic or using cocaine, it doesn't happen. More likely (much more likely!), the new workouts have made the chest muscles sore and that's the cause of symptoms.