There is no Action Replay code to instantly age dogs to adults in Nintendogs: Dalmatian & Friends (DS). The aging system is tied to real-time clock progression and trainer points—both stored in encrypted save sectors that AR codes cannot reliably modify without corruption.
Nintendogs is designed around daily rituals: walk, feed, brush, teach "Sit," enter disc competition, find presents. The bond forms over weeks. Rushing to "adult" skips the entire game.
If you want a finished save, search "Nintendogs Dalmatian 100% save file" for use with a flashcart (TWiLight Menu++)—but you'll miss the journey.