Ultimately, "trade" is defined by social custom. Another term is "profession." It probably makes more sense to look at how different occupations are trained. Some training is defined by licensure requirements, possibly union standards, and so forth. The duration of training varies by occupation, e.g., medicine takes longer than truck driving. Some training occurs in a college setting, such as medicine and nursing. Some is provided by local or regional trade schools, or by the military.

An interesting thing about programming is that we still haven't reached a consensus on what kind of training is needed. So we can say it's a trade, profession, or whatever, but that doesn't tell us anything useful about it.