Twerking is a grown-up activity ... so says Wu Tang Clan co-founder RZA  ... who tells TMZ, there should be an age requirement for booty shaking.

RZA was leaving Katsuya with his family last night -- when we asked about the twerk trend taking over the country.

The hip hop legend told us he'd like to see kids hold off on the adult activity and enjoy their childhoods -- telling us, "Don't rush the childhood ... it's the best and shortest part of your life."

Meantime, here's Miley Cyrus ...