The line mentioned is this:

RED XIII: "Please stop, grandfather. I'm 48."

Bugenhagen's response to that is:

BUGENHAGEN: "Ho Ho Hoooo. Nanaki's tribe has incredible longevity. So you see his 48 years would only be equivalent to say that of a 15 or 16 year old in human reckoning."

So in other words, he was born 48 years prior to that conversation, but that doesn't mean he's old yet since his species live a lot longer. In fact, he's only around the "teenage" of his species.