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!