Jessica Simpson's drivers license says 37, as of today, but you could never tell it from her ass ... which she put on full display.

Jess spent at least part of her birthday tanning her cheeks in her pool, while sipping on what looks like a fruity adult beverage. She captioned this piece of art ... "Kiss My Butt 36." Suddenly wish your name was 36? 

Happy birthday, Jessica -- hard to believe it's been 12 years since ya slipped into the Daisy Dukes ... sure doesn't look it!