[Kaderin Triste]

Depending on the lore you follow, dragons can live anywhere between 150-1,200 human years. As such, it is hard to know exactly how long his infancy stage will last.

As his irl self (from a realistic/ imaginationless viewpoint) is just a plushie, that version of himself will never grow. On Trisphee? Maybe, but that's mostly up to the staff artists and item makers. I artwork? Sure, it would be cool to see Derpy drawn as older in future/later artworks.
But will he age and mature despite his small stature? Of course. Over time.