Zack didn't notice any developmental difference between him and his sister. Then again, in 22 years of babyhood, the difference of two years was negligible. Zack waddled his freshly-diapered butt into the living room.
As he entered, he felt a rumble in his gut. Suddenly, and without thinking about it, Zack squatted and released the contents of his bowels into his disposable diaper. His mom sniffed the air and got up from the couch.
"Does my little birthday boy need a diaper change?" She asked. Zack simply nodded, sucking a little faster on his pacifier. His mom led him to his room, which was now a nursery like Susie's, and helped him onto the changing table. From his experience changing Susie's diaper earlier, he knew exactly what to do during the change, and once the last tape on his teenager-sized diaper was sealed, he hopped off the changing table and went back to the living room.
He and Susie sat on a blanket on the floor, extra-large versions of babyish toys scattered around them. Meanwhile, their mom sat on the couch and watched TV. She was watching a documentary about elephants. Zack heard that elephants are dependent on their mother's milk for the first four years of their lives. That gave Zack an idea. He decided to make teenagers like him and Susie still be breastfed.
Next thing he knew, he was suckling on one of his mom's breasts and Susie was on the other. Zack was in love with the flavor and was filling himself with the milk. Once he was full, his mom burped him. Zack yawned, as did Susie.
"Looks like my babies are ready for bedtime!" said their mom. A few minutes later and Zack was tucked in in his new crib and the lights were off.