Well, short answer is that it was not "mostly against youth and old people". That only started occurrring within the city itself after the main defenses fell. The battle for Berlin took place over a 2 week period and took place in, and around the city, with various German armies defending and counter attacking.

Here's a decent summary of events: http://militaryhistory.about.com/od/worldwarii/p/World-War-Ii-Battle-Of-Berlin.htm