I'm in my mid-thirties and generally prefer girls in their twenties for conversation, but I've seen plenty of 18-20 year old SP's and had a great time. One of my ATF's was 19 years old and one of only two girls I ever repeated with five times. Reminded me of a high school girlfriend.
None of them are very good at judging age and when I've asked them to guess my age, most have guessed that I'm mid to late twenties. I used to take this as a compliment, but I know that girls closer to my age guess my age about right. In my experience, most of them just look up to you as the cool older guy and want to do anything to make you happy. The conversation is sometimes simple and some are pretty ditzy, but still cute, playful, inexperienced and (relatively) innocent. As long as they're legal and willing I have no problem.
I'm not that old that I can't relate to them and I have no problems with this, but it's definitely harder to relate to them then a 25+ year old. I'm not in this hobby though to have stimulating conversations, I can do that with the girl I'm dating. If that's what you're looking for go find a girlfriend and if you think 19 year old girls don't enjoy sex, you're very wrong. As long as they're friendly, fun, good looking and have a good attitude, I'm happy and at that age, most of them look good.