Found this interesting story that supposedly the name Kayleigh was made up by a dodgy old 80's prog rock band called Marillion as the title of their one fairly famous single around 1985. The song was lovesong about the lead singers girlfriend at the time and the name Kayleigh is derived from hers. Her first name was Kay her middle name was Lee, a bit of a fiddle with the spelling of the middle name, bung them together and bingo - a hit single and an original name which has since entered the culture and popularity lists. Explains why there seems to be loads of 20 year old Kayleighs punting around these days but i dont recall meeting a 30 year old one.
Whether its true or not i dont know but heres the interesting part (at least i think it is) the link at the top is for a quest to find the oldest Kayleigh out there. The best so far is 20 years old which would seem to justify the claim - Prove them wrong if you can.