It's a convenient shorthand for talking about and summarizing continuous statistical distributions. Shorthands are okay. As to your question, if you draw the li

It's a convenient shorthand for talking about and summarizing continuous statistical distributions. Shorthands are okay. As to your question, if you draw the lines between differently, you only get slightly different results, because it's fundamentally measuring the dependence of various factors on age, and the exact way you draw those lines isn't important.