Pokémon aging

Comparison between Ash's Lapras's younger and older appearances

Pokémon aging is occasionally shown in Pokémon media. This includes appearances of young and old Pokémon. Aging is usually separate from evolution, as unevolved Pokémon have been shown to visibly be older without evolving.

In the core series games

Pokédex entries

Some Pokédex entries mention how a Pokémon changes as it grows up:

Squirtle's shell forms and hardens after it is born.
Vulpix is born with one tail, which splits into more tails as it grows.

In the spin-off games

Pokémon Puzzle League

In Pokémon Puzzle League, Tracey's elderly Scyther is one of the Pokémon in his team.

In the anime

Original series

Puka is an old Pikachu that appeared in The Pi-Kahuna. He also appeared younger in a flashback from 20 years before.
Ash's Lapras debuted in The Lost Lapras, where it was caught as a young Lapras. In Viva Las Lapras, the Lapras herd is seen, including the mother of Ash's Lapras. In Lapras of Luxury, it was revealed that Ash's Lapras eventually matured into an adult and became the leader of the Lapras herd.
Tracey's Scyther, which debuted in Tracey Gets Bugged, is shown to be elderly, losing the leadership of its herd to a younger Scyther.
Silver is a baby Lugia that debuted in The Mystery is History. Silver's parent is a Lugia that debuted earlier in Around the Whirlpool.
A Noctowl having an old beard appeared in Throwing in the Noctowl belonging to Wings Alexander. In flashbacks, this Noctowl was seen as a Hoothoot and as a young Noctowl.

Pikachu shorts

An elderly bearded Bellsprout appeared in PK13.

Pokémon the Series: Ruby and Sapphire

An elderly Treecko appeared in Tree's a Crowd.

Pokémon the Series: Diamond and Pearl

Newly hatched Psyduck appeared in The Psyduck Stops Here!.

Pokémon the Series: Sun & Moon

An elderly Stoutland debuted in That's Why the Litten is a Scamp!‬, taking care of a wild Litten. Uniquely for a visibly old Pokémon in the anime, this Stoutland's age eventually caught up with it, causing it to pass away following the events of One Journey Ends, Another Begins....
An artisitic depiction of a baby form of Celesteela appeared in Rise and Shine, Starship!.
A baby Miltank appeared alongside multiple Miltank in The Young Flame Strikes Back!.