My little brother is four years younger than I am. He’s come to me with fairly practical sex questions on a few occasions, and we’ve talked pretty openly about things that probably wouldn’t be suitable for the dinner table, heh. It doesn’t bother me, and it makes me happy to know that he is comfortable coming to me for advice and support when he needs it. Frankly, I’d rather he come to me than go to the internet or his crazy friends - Og knows what he’d pick up there! I can give him science! 
HazelNutCoffee - I’m glad you expanded on your OP, because the clarification speaks really well of how you handled the situation. Good on ya for both setting a subtle boundary and providing on-the-spot support. Might I suggest having a plan in place for next time, though? If he doesn’t have a support system for these kinds of questions, it will probably be really helpful for both of you if you have a) a plan for what you are and are not willing to discuss with him and b) a plan for what to recommend to him for the things you are not willing to discuss, resource-wise. Guys, generally, are very solution-oriented; if you can’t help him, being able to point him to somewhere that can is often quite useful.
May I recommend (as reasonably good, informative sites which offer sex-positive, safer-sex geared information):
www.sexetc.org
They are both aimed at the just-graduating-high-school crowd, but offer very good basic information for young adults just getting into sex and wondering what it’s all about and whether they’re “normal”. 
