I think the pet aging system is one of the features that makes Grow A Garden more strategic than it first appears. At the beginning, I mostly focused on getting new pets, but over time I realized that managing pets at different life stages can have a huge impact on farm efficiency.
I've found that keeping a combination of Adult and Elder pets works best for maintaining steady resource production. Planning these combinations can sometimes be tricky, so I've even started using a grow a garden Calculator to compare bonuses and decide which pets are worth investing more time in.
It would be great if future updates added even more unique traits for Elder pets, since that would make long-term pet management even more rewarding. Overall, the aging system definitely encourages experimentation instead of simply collecting as many pets as possible.