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Not always, but they typically tend to be young and some of the Japanese names tend to imply that most unevolved mons outside of a gameplay standard are younger. Aron and Ratatta are named Kokodora and Koratta, while their evolutions are Kodora and Ratta, essentially making their names something like "kid Ratta". Taillow's dex entry also says that they're young and just left the dex, but the dex shouldn't be taken all that seriously. There's also the baby Pokemon, too.
So basically, some of them are designed to be like a younger Pokemon, but they don't outright determine whether a mon's older or younger.
