When I was 25, I was pretty shy and never dreamed of complimenting a complete stranger. Today at 50, I compliment all the time, including women half my age, but do so very respectfully and for no other reason than to just make someone smile.
Here's a example:
"I'm old enough to be your dad so please know that I'm not hitting on you, but I have to tell you, you're smile lights up the entire room. Make sure you always share your smile with someone in need."
This always proves to be a great experience because everything I say is true: I'm old, I'm not hitting on them, their smile is awesome, and its important that they always share a smile with others.
And for the record, even an average, ho hum, smile can instantly become a person's best feature when you've brought them a moment of happiness.