Nami always calls Sanji, "Sanji-kun"

Is he younger?

The suffix 'kun' isn't always about age. Naturally it's hard to translate, but it's sort of like indicating someone who is of relatively equal social standing but isn't close enough of a friend to drop the honorific. 'kun' is often used by girls when addressing guys in school, and always with the family name.

Similarly, 'chan' isn't always age specific either, even though it typically represents a condescension or affection for something smaller/weaker/younger than the speaker.

'Senpai'/'Sempai' is much more specific as a form of address to one's social superior and typically indicates the speaker is younger, but not always.

None of these are all that easy to nail down which is why you'll usually see them untranslated in fan subs. In official subs, usually the translator will go with a contextual 'translation' and use the character's first name if a girl is saying surname-kun, because using someone's surname in English has a totally different connotation.

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Charan wrote:

but it's sort of like indicating someone who is of relatively equal social standing but isn't close enough of a friend to drop the honorific.

That doesn't make sense to me, since I'm pretty sure they are "Nakama"

Nakama means 'comrade' or 'ally' really. Companion, perhaps. The implication of 'nakama' is that the two (or more) of you are facing some sort of challenge together. It's not quite 'yuujin' or 'tomodachi' (friend) and probably not as intimate as 'aibo' (partner in crime, so to speak).

edit: PS I'm not trolling. I actually do understand a fair bit of Japanese. Just in case you weren't sure.