Pretty sure the starting back 4 for Juventus are all over 30 - it is definitely an ageing defence, although it's vastly experienced which will negate the age factor in the short term.
Crazy for Juve to sell Mattia Caldara to AC Milan - a solid young centre back, but I would not be surprised if they had a buy back clause for him.
At the end of the day - if the age of the defence does eventually become problematic Juventus can just go out and take their pick of the best young centre backs in the league.