Subscription interval based on milestones

We are selling boxes, and the boxes should be shipped based on the dog's age. When customer place an order he will receive 1st box. We have now milestones based on dog's age, so if the dog is 5 weeks old the customer will get next subscription on 8th week and the another milestone will be reached at 25th week, so he will get invoice on 25th week. We have several milestones.