Portfolios can be done over time, sometimes we get people with beautiful portfolios that they put together in college. But when it comes to a work environment, you don't have 4 years to finish a poster. You have less than 4 days!
Portfolios can be nice to look at, but rip them apart, find mistakes, find errors!
What's important is not what looks right, but finding what is wrong!
Experience is a good indicator, but likely if someone spent 15 years in one place, was it because the environment was not fast paced enough, not enough work, what are your expectations here in terms of turnaround. Has someone with 15 years in one place got enough experience working to your pace?
Similarly, I'd be wary of people who have been 6 months in one place, and 1.5 years in another, and 3 months in another place - how long will they stay and work with you?
I'd be likely to explore the portfolios, ask how long a project took, ask how many people were involved in the piece, did they come up with the concept, or was the artwork bought/supplied like that and they made a few edits?
Experience, find out why they are leaving their current role, why after 10/15 years they are moving on, similarly if they are constantly changing jobs find out why this is, what is motivating them to keep moving to another role.