Also keep in mind that your brain does not learn the same things at the same age. When your brain first forms and starts operating (< 18 days after conception) it will start learning very fast relations. It learns about the high frequency components of the human body, presumably things like how the electrical signals for a touch look like, or what the carrier signals are for your eye and the like. This also results in very, very fast sleep-wake cycles. When you "wake up from nothingness" so to speak, the cycle is less than a second. By 2-3 months it's about 5 minutes, and as any parent will know and complain about, at birth it's 2-3 hours. Obviously it eventually synchronizes to the sun, but that takes more than a year.

Then as you mature, the myelin sheath starts to grow, which "locks in" the high frequency stuff it's already learned. By the time you're born you've lost the ability to learn about events that takes less than 3-4 ms to complete. By the time you're twenty, it will be at about 50ms. This is why people who start learning a sport at 3-4 years at most effectively have an insurmountable advantage.

This has all sorts of effects that you will recognize in a growing human being. Short-term memory starts using progressively longer frequency signals to store itself. Reaction speed for anything new goes down (reflexes, and trained behaviors learned earlier stay in force though). Your ability to reason about things far away massively increases (a 4 year old kid cannot plan more than 20 minutes or so ahead without adult help, which mostly consists of reminding them what they were doing every 10 minutes or so).

Furthermore the way your brain compresses signals is by distributing itself. There are small parts of your brain everywhere in your body, and several big blobs in your head. Every muscle has a cluster of neurons on it, and things like the heart, bigger muscles (like stomach muscles, biceps, triceps, ...) have large clusters on them. Those are clusters that do the same learning process as the rest of your brain, they're not preprogrammed. So your brain does not directly control your body, it negotiates what should be done with those local clusters. Because of this your body is able to have very short feedback loops controlling your muscles despite the fact that the decision process itself takes tens of milliseconds (and happens 1-2 meters away from the actual muscle). Now these clusters start out as blank as the rest of your brain, but they grow together, and they learn to communicate together. Different people send signals differently (well, the basic structure of the signal is the same, the semantic meaning, such as which muscle it affects, what timeframe it's talking about, ... is not). This is why if you connect a robot arm to a human, they cannot use it for fine motor control, even after months of practice, it improves to a pretty pathetic level, and just doesn't improve further (certainly not enough to write at normal size with it). If you want to be pedantic, there is more than one level of indirection. The first is the "brain region" -> spinal cord interface, and then every 15 cm or so in your spinal cord is another layer of control, and every point where neurons split up is another control point.

Here's a rather basic introduction : http://en.wikipedia.org/wiki/Motor_control#Sensorimotor_feed...

All this means that your brain's lower level body control is effectively a ROM by the time you've grown up, and it is very dependant on tiny variations in the length of neurons to various parts of your body. Every single human body has a custom protocol for controlling the muscles that's different from everyone else's (so it's not possible, like in the movies, to mount something on the spinal column and take over motor control). Connecting up a brain to a new body, assuming you can keep it stable until the person wakes up, will immediately lead to a seizure (because the brain will be sending wrong signals to the heart), extremely rapid breathing, violent convulsions, ... until the patient is dead.