Some view it as an exciting transition, while others would prefer to remain perpetually in their thirties.
Luckily for Matthew McConaughey, he is not in the latter group.
On Wednesday the Texan celebrated with the outlook that life can only get better.
"The 30's were good to me and I'm really looking forward to my 40s, 'cause I'm a late bloomer," the Sahara star wrote on his MySpace page.
"Got everything I need for a perfect 40th… a happy and healthy lady, son, and another baby on the way."
So just how did the Hollywood heart-throb celebrate such a momentous occasion?
With a big party the night before that went on until 4.30am featuring a surprise performance from one of his favourite musicians, country star Jamey Johnson.
"There were some epic stories told, lots of laughs and definitely some roasting," he wrote.
As for the big day itself, Matthew said: "Going to spend some more time with my family and friends today, grillin' steaks and sippin' beers, celebratin' more to live for…"