Pokémon aging

Pokémon aging is occasionally shown in Pokémon media. This includes appearances of young and old Pokémon.

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

In the core series games

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

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

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 has an elderly Scyther that debuted in Tracey Gets Bugged.
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.
Wings Alexander is an old man with an old bearded Noctowl who appeared in Throwing in the Noctowl. In flashbacks, he was also seen with his Noctowl as a Hoothoot and as a young Noctowl.

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....
A baby form of Celesteela appeared in Rise and Shine, Starship!.

Jaye, an old man with an old Hoothoot, appeared in Finding a Legend!. Jaye has had his Hoothoot since his youth.