Anyone can be a polymath as long as he or she has the right motivation.
A polymath is not necessarily a brain. In fact, a polymath usually does not think of his or herself as being particularly smart, only curious. Curiosity and interest are the true motivation for work, both intellectual work and the nitty gritty of hands on inventing. Thomas Edison said that genius is 99% perspiration and 1% inspiration. He had a passion for getting his hands dirty, for tinkering, for inventing through trial and error. The polymath makes lots of mistakes. This is how new sciences are created.
The brain requires constant exercise and continues to develop throughout life. The polymath tries to exercise all parts of the brain. It is more than just acquiring general knowledge, not essentially memorization.