Learning how to write with your weak hand, learn a different language, learn the new way to ride a bike. It opens up new synapses in your brain. They are all known things, but they don't come naturally.
The man is wrong, it is teaching your brain something new, and yes it is easier as a child, but you CAN learn to ride the new bike. Like a foreign language or left hand perfect writing, it'll take a while, but the alteration of a skill is very important as you grow older. It increases brain activity.
Here is a fun one I did last year that actually has led to some of my more recent theories on prime and reimann's sum: add another digit after 9. instead of 0-9, now it is 0-X.
1 2 3 4 5 6 7 8 9 X 10 11 12 13 14 15 16 17 18 19 1X 20
2*X=19 5+X=14
it is weird, but it works out the same way, but all the principles of math are a little funked.