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It's a toss-up at this point, since they have failed attempts with the both the classic late 30s/early 40s iteration and the Ultimate Universe early 20s iteration.
They can try to keep skewing younger since the Ultimate Universe is a huge influence on the MCU and they need a new generation of characters to pick up the slack of the original Avengers who are all leaving. The hiring of Jon Watts, notable for most teen-oriented Spider-Man movies ever, could be evidence that that's the plan.
Or they could further dissociate from Tranktastic Four and earn points with the fans in the process by going back to the classic 616 version, maybe de-aging them a bit into the more marketable early-to-mid 30s age range. The fact that 2005's Fantastic Four is generally more well-regarded than Fant4stic and even has some nostalgia going for it could be extra incentive.
It could go either way.
