A Pokémon spends most of its time in a Poké Ball, but does it physically age during its time there, or is its physical condition frozen until it's called on in combat? It would have been easy for the Pokémon franchise to never directly address the subject of death, but Game Freak decided to jump in the deep end, starting with Lavender Town in Pokémon Red & Blue.
Lavender Town is the place where Pokémon are buried, and places like the Pokémon Tower are filled with people in mourning for their lost companions. The Pokédex refers to Pokémon hunting each other for food, as well as humans eating Pokémon, with Farfetch'd and Sharpedo being two notable Pokémon breeds that have ended up in stew pots.
The Pokémon anime has shown Pokémon of different ages, with Tracey capturing a noticeably older Scyther during the Orange Islands seasons. The Sun & Moon seasons also have a storyline involving a Stoutland that passes away due to old age. The Pokémon video games don't have a mechanic for age, save for a loose interpretation of breeding Pokémon and baby forms. This raises the question of whether the Poké Ball slows down the aging process for Pokémon when they enter them, as this could act as an in-game explanation for Pokémon not aging.
Poké Balls Are Like Hotel Rooms For Pokémon
There is actually a canon answer to what's inside a Poké Ball. Longtime Pokémon composer & director Junichi Masuda told Kotaku that the inside of a Poké Ball is the equivalent of a fancy suite in a high-end hotel. This suggests the Pokémon are relaxing inside their Poké Ball and still aware of their surroundings while they wait to be called on again. The mechanics of this location aren't explained, as the inside of a Poké Ball looks empty, but if Poké Balls can be digitized and entered into a PC, then what's to say that the same process cannot happen on a smaller scale for the Pokémon themselves?
There's also the question of poison damage to consider. When a Pokémon is poisoned in battle, they continue to take damage until healed with an item or taken to a Pokémon Center. The damage occurs every time the player takes a step. Pokémon's breeding mechanic is also tied to steps, suggesting that this represents a passage of time. If the Pokémon's physical state was suspended inside of the Poké Ball, then they shouldn't take more damage once the battle ends. It's safe to say that Pokémon continue to age in their Poké Ball, even though this doesn't have a mechanical effect in-game.
Source: Kotaku