Can someone be diagnosed with bipolar as a teenager even if he didn't show any symptoms as a child?

Sure, it pops up: Bipolar can express itself usually in the mid to late teens, or into the 20's. It can be seen earlier, but after puberty is more common. Often though, once there is full expression, people who know the patient well can look back and see "moments" where things weren't right but they were explained away. Most mental illnesses are like that.

Yes: Classical symptoms of mania or depression typically begin from the mid teens to early 20's.

It is.: Yes, bipolar disorder can manifest in adolescence without prior childhood symptoms although it can also be common to have "sub threshold" baseline symptoms that don't meet illness criteria.