Turning 60 is a major milestone, as many people see it as the beginning of their life's last quarter. However, that doesn't mean birthday wishes can't provoke a smile: "Happy 60th birthday! Sure, you've been around the block a time or two, but you look GREAT doing it!"
Another option is to play with the idea of the person reaching vintage status. "Lots of wines get better with age. A few people do, too. Congratulations on making it to the big 6-0! You're a rare vintage indeed!" Of course, letting someone know that being 60 doesn't mean they aren't sexy can work, too.
BirthdayMessages.net suggests: "Happy 60th birthday! 60 years ago, you were a baby. At 60, you’re simply babilicious." Acknowledging that the birthday celebrant is hitting a major milestone while keeping things fun, light and fresh is key. Referencing something from their past, like a pop song that dominated the charts or a celebrity who was big when they were growing up, can work to illustrate the passage of time in a light-hearted way. This approach requires some creativity and time, but the recipient will know the wishes you're sending them aren't generic and have been crafted with them in mind.