According to Edric, he’s 12 years old (III: 493)
It seems likely that Mormont was not a skinchanger consciously. However, his affinity with his raven, and the raven’s precocious vocabulary, suggests that he may have had at least some of the talent. The raven’s key role in the election of Jon Snow, in particular, is striking (III: 897) in how the raven seems almost preternaturally capable of moving the Watch to vote for Jon. Obviously, someone else must have placed him in the kettle, but his calling out, “Snow” and perching on Jon’s shoulder are either remarkable coincidences or an indication that some spark of Lord Mormont lives on in the raven.