35, or 34, depending on when the extension is done, and if he's a QB worth that money now and doesn't suffer an injury, he will get a big extension, even if it's only a 3-4 year big deal.
However, if your case is that he's washed up at 35, then it's even worse, because then we will be looking for another QB in the free agent market when they are super expensive.
Bottom line, it's silly to make a case for spending big money now, that it will prevent needing to resign a rookie to even bigger money in five years, after both the rookie and cousins contracts will be up.