90 days really isn't a magic number for Stilton. Just because you age any Stilton to 90 days doesn't mean its automatically awesome tasting. Imo i dont like this generalization of aging time. You also have to consider the size of the cheese when it comes to aging. What if you made a 2 gallon batch? If you do age it to 90 days, its either going to dry out, ammoniate or both. Cropwell Bishop start grading their Stilton at 6 weeks to determine if its aging properly. They sell their Stilton between 8 to 11 weeks of age and their Stilton weighs 8 kg. There is no magic number in aging imo, it really depends on the actions of the micro organisms, temperature and environment.
Like Tomer said use an apple corer to check for veining and the most important thing to do is eat some of it. If its up to your standard, cut it open and enjoy. If it tastes funky, aging it longer wont make it better.