Do it in reverse. 15-20 first, the next generation, is 10-15 etc...I've always wanted to run a campaign in which the first round of characters retire at 5th Level. The second round of characters start at 5th Level and are the children / mentees / students etc of the previous characters.
The second round of characters retires at 10th Level, and then a third round of characters starts at 10th Level as the next generation.
Each time the characters retire, the campaign world moves 20 years or so into the future.
I think it would be a really epic campaign!
Yes. In a yearlong campaign, where the DM told us prior to character creation we had to have someone important to us go missing, I created the typical Human Champion Fighter with a background of Outlander. (Two things: We found out it was a cult, and at the time, I do believe there was only the PHB available. No supplemental books had been released yet.) I was a logger, who came home and found my two twin girls gone missing, and my wife dead. Nothing crazy inventive, but the trope worked great for the campaign.As the title says.
I'm not talking about writing an ending for your character when the campaign is over.
Have you ever retired a character and made a new one instead to continue playing?
If so, why?
I've only ever retired a character once, (that I can recall) he fell in love with an NPC and retired from the adventuring life.
I've put this in a DnD thread, but feel free to mention other systems as well.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.