The staff of Pokemon Dreams often gets asked 'Is one year young for a pokemon?' or 'Is 35 middle aged?'. We encountered these questions so often that they've merited a news file of their own.
Like many things on Pokemon Dreams, you need to look to the real world for your answers. If someone walked up to you and asked 'Is 5 young or old for an animal?' what would you say? It totally depends on an animal. For a mouse a year might be old age, but for a horse a year is still immature. Same thing goes for pokemon. When you need an idea of ages, look to that animal type. Is a year old for a Rattata? Well, what is a year-old rat like? Is 10 years old for a Ponyta? Look to a horse.
Remember, this is only a guideline. Pokemon are like their RL animal counterpart, but they're not the same exact thing. In general, and especially for the shorter-lived creatures, you take the RL number and add to it. A medium to large sized dog will live 10-15 years in the real world, so a Growlithe might live to 20 or so. If you go to a larger/longer-lived creature you add less. A real life horse might live to 30 (if it's lucky), so you'd add little to nothing to that number to get a Ponyta age range. Going the other way, a butterfly won't live a year, but a Butterfree could still be around 5 years later.
So to sum it up, everything tends to live at least a few years, and some things live a good many years. Bug pokemon would have the shortest lives, followed by small animals that burn their energy quickly (Rattata, Nidoran), plant pokemon next, then most animal-pokemon, and longest living would be creatures like Lapras.
If you need this information for your application, remember that it's perfectly fine to reply with 'Young', 'Mature', 'Older than dirt' on the age question. You can add an age in years if you like, but for a pokemon that is not required.
If you have a question or need more information, just contact a staffer.