Well.. I've done that with an uncle when I was younger - my uncle is just 5+ yo than me, one of my best friends btw.
But, more relevant, at school I practice Taekwondo for a couple of years, at some point our trainer just ask if we wanted to take some naked lessons, there weren't girls at that time, so he offered that. I supported it because it made us feel more confident, avoid to judge, etc. Also it was harder, with more rules (respect, self respect, do not kick other's balls haha, etc.). It worthed it.