The head writer RikuSanjo originally wanted Blue to be a 45-year-old family man. The idea was vetoed, but some elements of it seem to have been used nonetheless, and it was even given a reference in Episode 8 when KyoruyBlue's "old man jokes" cause Yuko to speculate on his age:
Yuko: He might be a hero, but he's got to be at least 40!
Nobu: No way Blue is already 40 years old! He's definitely younger than that. ...I think.
What Measure Is a Non-Badass?: The creator realized that after the majority of the previous series have Yellows who are female. Predicting that the target audience would consider Kyoryu Yellow too feminine regardless of gender, yellow was ultimately excluded from the main team.