How old is a puppy when he starts "marking?" Bentley is 13 months and size-wise is full-grown, at least I hope - he's almost 25 lbs! He never marks, and he still just stretches out to pee - he doesn't hike his leg. Our daughter's 6-year old Bischon goes around marking everything. Hubby & I both grew up on farms with only outside dogs. They just did their business and nobody noticed or even cared how or where; consequently, at our advanced age we're novices at this indoor dog thing! Hopefully, Bentley will not be like his "cousin," Elvis, the Bishon!