As a very rough guide, small-breeds most frequently mature between the ages of 9 and 12 months young. It can sometimes be a tad tricky to tell whether your Dachshund is growing because of muscle gain or weight gain at first, so we suggest you start looking at a body condition score chart when your pup enters this age range. It will show you how to tell if your Dachshund is simply becoming heavier because he’s growing longer, or whether he’s now getting too many calories from a puppy formula.