just ask yourself, do you know them pretty much inside out? do they know you inside out? if so then talk about it before doing it else you'll likely end up breaking up at SOME point and it would just be all awkward
Tell your friend that you value your friendship and getting into a relationship would cause the friendship to end if the two of you were to have a bad break up.
Best friends make the best person to be in a relationship with. If you do go out, make sure that you both understand the consequences of the possibility to having it end, and work on never letting it.
how long have you two been friends?
well if he is your age, he might have liked you for a long time so he might just want to have sex with you.