Aside from their origins, I don't think Subaru and Ginga were being trained in combat at a young age, since Subaru was a clumsy crybaby when she was little, only deciding to actually learn Shooting Arts after the airport fire incident. It was also stated that Quint busted the laboratory before any further research could be made, thus implying that Subaru and Ginga were rescued before they could be tested and trained for combat. TSAB doctors probably found Subaru's rather dangerous ability during one of their tests.
Besides, she grew up having a very supportive family. Compare that to an orphaned Teana, an abandoned Erio, and an exiled Caro. She had it easy, she's just very emphatic and understanding about others that she gives them her support in any way she can.