Juventus is run very professionally. New contracts and wages will be offered according to the club's own policies which take into consideration age, level of skill, growth, merits etc. Juventus is not out to "screw over" any of their players. The club will offer a better contract to Pogba when the time is right according to the club's policies. If Pogba is happy with being part of the wage structure of Juventus FC, there will never be a problem. The day he wants more money than the wage policies dictate, the club must decide whether or not to make an exception and pay him what he wants (or close to what he wants), or let him go. It's that simple, really.
As for when he could ask for more money, he could do that whenever he wants. But as for now, the terms and conditions of his current contract (the one that runs until 2017) is what counts.