Here's the thing. For simplification purposes, we tend to assume that there's equal distribution among age groups.

In reality… IT IS NOT TRUE. Usually we talk about the age pyramid, not the age cube, because in most countries (and in nature) the “base” of the pyramid tends to be larger.

The fact that the population is growing suggests that the base of the pyramid may be larger.

So it is easier and wiser to assume that the base includes it's “fair share” of people (equal distribution) + that 4% of population increase.

Hi there,

Just a few things to add:

• Good thinking on growth (it's a good thing to flag to interviewer!)
• Cases almost always have some form of “over” simplification
• Remember that you might need to consider/think about age splits in countries where there is absolutely a big difference (Japan, India, etc.)

I see you already received a few useful answers, so I'll only confirm that the flat distribution is quite common when looking at ages. 

Of course, in practice, you don't have the same amount of people born across decades. 

Sharing with you an additional resource that you might find useful for casing, in particular around maths - a list of the most common terms that show up in interviews:

• Expert Guide: The Must-Know Consulting Terms for Interviews

Good luck!
Cristian