The 'average age' of the class is probably like 21-22 in second year, but obviously that's because the majority of the class get in through HSFY, HSFY'ers typically being teenagers straight (or near enough) out of highschool.
The graduate entrants are generally only 2-4 years older, the vast majority of the graduate entrants come straight out of a BSc/BBioMedSc/BA/other, +/- hons.
The supergrads ('other category' entrants) can be more or less any age. Most are probably 25-35, but there's the occasional one in their forties.
(This is all for Otago, Auckland doesn't have an 'other' category, so I have no idea how the older entrants get in there)
I'm fairly certain the interviewers aren't allowed to (openly) directly base their judgement on age, but I'd say that if you're much over thirty five - forty then you're going to have to justify why the government should spend a lot of money training you for a career you'll be in for 20 years less than an undergrad entrant half your age. They'd probably also expect a good explanation of why you'd like to throw yourself into such a lengthy training programme at that point in life. There are stories floating around of entrants in their fifties, but they're very rare, and giving spots to them is a wee bit controversial.
tl;dr, the interviewers can't officially directly mark you down for age, but I'd wager that if you're >35 then it'll probably work against you unless you've got some good answers to any age-related questions they might ask.