I've had quite a few, it's hard to pinpoint the first time I was really interested by words, but I can call a few smaller ones. I read in an essay by Isaac Asimov the etymology of the word delapidate, and was completely fascinated by how the word means something different yet somehow the same.
I've been thinking about a comment mollusque made the other day on euryvocalic. He was able to remember an epiphanic moment at which words took on a new meaning for him. I think mine came from much more lowly circumstances. At about age 7 or 8 I read a little comic included in a Bazooka Bubblegum package that posed the question "What is the longest word in the world?" The answer, of course, is smiles, because there is a mile between the two s's. I remember thinking about that for days--it changed the way I thought about words and language in ways that are still with me today.