This is a trope I've seen in fiction as disparate as adventure novels (The Count of Monte Cristo), light comedies (The Importance of Being Earnest) and Dickens novels (Bleak House).
On a related note, how common was this in the early to mid 20th century? It's also a trope in fiction of that era too (Batman and Robin, for example?)
Similarly I'm curious about the same question, but more for the time period between the 10th and 13th century in Europe. Was the adoption of wards common among the wealthy and the nobility, and what was the common motivation for doing so? Political? Prestige? Benevolence?
Was it a common practice to take a political hostage as a ward after conflicts or as a means to prevent conflict?