My experience is probably not unusual: I remember trying on my mother's underwear two or three times when I was 8 or 9 (once, memorably, almost getting caught!), and dreaming about wearing the school uniform for girls (a cute plaid jumper!). But I really didn't dress until adulthood.
What triggered it for me? About 20 years ago, my wife urged me to put on some make up. (Actually, she had prodded me once before to do it, but I'd said no.) This time, I put on mascara, eye shadow and lipstick -- and it set off a flood of emotions and desire that has never really stopped. From that night on, I've worn panties regularly and other pieces of clothing with varying frequency.
Has anyone else had a trigger in adulthood that set off some dormant desires?