Age up potions are currently untradeable due to how they are earnt. Age Up potions can be earnt through the core gameplay loop of completing needs on pets past their fully grown stage, through the friendship bar. This makes them similar to Bucks and they would start to become a secondary currency.
Similarly to how we do not presently allow bucks trading for the reasons of not putting a numerical value on pets; age-up potions would serve the same function.
While we do not consider changing the tradability status of the age up potions at this time, it is not out of the realm of possibility and we may one day change our stance on this as seen with untradeable items previously.