Adcock isn't young, but he isn't old; he might captain for five years or more. He won't be good enough to make the national team, but on last year's form he was good enough to justify his selection for the state. This is a key reason for overlooking Graham Manou, who is slightly younger and has more captaincy and VC experience. Manou's batting isn't able to justify Pura Cup selection and he may yet be on notice.
Another element is reining in the riffraff. Cosgrove is known to have disciplinary problems and it's likely Lehmann realises he's not the kind of person who brings discipline to a team. Considering much of the team is in their early to mid twenties, not only will discipline and experience be problems for a captain to work on, but those players effectively rule themselves out of captaincy. It's not hard to see what age bracket the SACA felt was a happy medium.
They say Adcock is a lawyer and a high acheiver, well, if there's one thing I know about my law friends it's that they're very good at getting their way.