Chiellini always had that 'extra' in him, which gives him leadership IMO. But it's his qualities that shouldn't classify him as a captain I think.
Marchisio never says a single word on the pitch, I think captain should motivate the team.
And I don't think it comes with age, some players have it inside them and some don't, simple as that. Some are captains even at 20s because it suits them.