[steadyelk]

It could be intentional. For a lot of folks, running the Boston Marathon is a dream, so maybe the BAA wants to make that dream just slightly more attainable the older you get.

[poutrathor]

As always it's probably because maths still is too hard for most people and keeping the rule simple won over fairness.

[votepaunchy]

Ironman just switched to age-grading, which is just as simple and already used by many runner training plans and race calculators.

https://www.ironman.com/news/age-group-qualification-system